Thanks for your input, but I have 6 locks so converting all to another brand/type is not an option right now.
Kwikset might not be the best locks out there, but all of my 6 locks had worked no problem at all for at least 3, 4 years now. Something caused them to act this way. Why all 6 have gone bad at the same time while they worked perfectly fine before?
Just a note, in my experience Kiwkset lock zwave modules tend to last about 4-6 years before they need replacement. Things like bad battery life, communication etc are common after a years. The good news is that the modules are replaceable. Bad modules can infact hinder the mesh by introducing noise or signal timing issues. I would recommend take out all the modules from your locks and then reinsert them one by one and see when the mesh breaks till you find the faulty module.
Is your lock within a few feet of the hub?
The locks are absolutely fine. It’s the Z-Wave card in the lock that’s bad! Most of them are the old Z-Wave and not Z-Wave Plus.
Yes sir. Both my phone and the lock I was working with are within less than 10 feet from the router and the hub.
Actually all my locks are Zwave Plus.
Have you tried turning off your main circuit breaker in your house for 30 seconds and then turning it back on? This will restart all your mains-powered Z-Wave devices.
anytime you cut all power to the house at the circuit breaker you risk damage to anything connected to it, even your non-home automation mains-powered items (The risk is small, but real, and magnified by a whole house restart), I am reluctant to ever recommend that as normal troubleshooting. I understand that it’s easier than going around and individually restarting devices, but it’s not risk-free. If nothing else, you probably reduce the working life of your individual surge protectors with each power restoration.
And Since in this case the primary issue is battery-powered locks, i’d be looking in a different direction anyway. @rboy ‘s suggestion to remove the lock modules one at a time as a troubleshooting diagnostic Sounds like a good approach to me.
JMO 
Turning off the main breaker is similar to unplugging everything in the house and plugging it back in. Electricians perform this procedure regularly. If the OP doesn’t have air gaps on switches, how would he reset those devices?
Very different in terms of surge impact. From the NEC guide to SWD standards:
If a circuit has a large inductive load on it and also has a device that doesn’t like voltage transients, tripping the breaker may cause the inductive load to generate a voltage spike which could damage the other device. The same could occur if both devices were plugged into the same power strip and it was unplugged from the wall, but would not occur if they were unplugged separately.
Also, in response to:
Electricians perform this procedure regularly.
But not over and over at the same house. And they usually tell you they’re going to do it ahead of time so you can unplug vulnerable items as desired. (Again, the risk comes from the surge when the power comes back on and many devices try to start up all at the same time. Not from the cut.)
If the OP doesn’t have air gaps on switches, how would he reset those devices?
(And again, none of this would have anything to do with battery-powered locks, which was the problem being discussed in this thread.)
The usual manufacturer’s advice from companies like Jasco is to do just one circuit at a time, not the whole house. It’s more effort, but reduces the possibility of surge damage.
He is saying he is having a problem with more then the locks!
I never suggested he should do this repeatedly! If he had an old appliance, maybe 30 years old, then yes, I would unplug it.
Actualy this would cause a brief sag in voltage and not a surge when everything is powered back on at the same time.
Typical NEC verbiage. (They are good at creating standards, but not great at writing about them. 
)
Although they’ve written the passage in terms of “unplugging,“ the problem they describe occurs when everything is plugged back in and all the devices are going through their startup routines at the exact same time. You can see the same problem after a Brownout and the power comes back on.
Thank you all for continuing to troubleshoot this. Sorry I am not as responsive as I should be to answer all of your questions, but I am trying :).
To answer question about shutting down power breaker. Most of my failed devices are battery-powered locks so that wouldn’t make any different. But to mimic that, I did try to remove batteries on these locks when resetting them just to make sure. I also unplugged the smartthings hub and removed the batteries for at least 10 minutes before I tried to add them again. So far I still haven’t had any luck adding the one I reset yesterday back. Will continue to try.
I also tried to reset/re-add the other devices (outlets) back. While they’re all back I did notice that it took a long time for the hub to communicate with them even though I plugged the outlet near the hub since I wasn’t able to reset/re-add it from where it used to be (before this problem, I was able to re-add any device from where they are). So it seems like communication is an issue?
As Alejandro said, there is a problem with your Z-wave network. This could be any Z-wave device in your network causing an issue—be it a plug, an outlet, or anything else. Since only mains-powered devices are repeaters, this is most likely your issue. The only way to try and fix this is to power off each and every device and then power it back on.
That’s great, but as I stated above, if there is a problem with your Z-wave mesh, then it’s likely a mains-powered device!
I see. I will try to shut down the entire house for few minutes when it’s doable and will provide update.
Quick question. If all of my AC-powered devices were online, would this still be an issue? Meaning would one or more devices stop “repeating” even though they’re online?? Or only offline devices could cause problem? Just curious.
Yes, It may be online but not routing properly.
I am very happy and excited to report that after reset the entire house’s power, all the locks (that were offline) came back online. Was able to also add the one I reset yesterday and everything is now working as it should (at least for now). Thank you so so much for all the suggestions and troubleshooting you all have done. It’s greatly appreciated. It’d been very frustrating last few months not able to control things remotely and not knowing why. I’ve learned so much after this incident.
One thing (that might just be a coincidence), but my Ring alarm system had also been acting up lately the same time my smartthings system went bad, and now it’s slowing coming back too. I guess Ring uses zigbee and because all the repeating devices were not working causing Ring communication gone bad too?
Anyway again thank you so much. You just don’t know how happy I am now :). Will report back if problem comes back.
HMT
Ring uses Zwave also! Im glad your up and running again!
That might mean nothing or it might be very important information.
As @mlchelp mentioned, ring also uses Z wave. the two networks don’t talk to each other in anyway, so nothing flaky about smartthings would’ve affected Ring. but an environmental Event might affect both. There aren’t a lot of things that run in the same frequency as zwave, but there are some.
These days It’s most frequently drones. You might have a neighbor, or even the city might have started using them near your house. Some baby monitors, and some of the old style cordless telephones also.
Again, not saying it’s related, but it might be.
I was reading some stuff I googled and found a few articles that state, ‘Other Z-Wave networks with unusually high traffic can also be a root cause of problems.’ Could the ST Z-Wave network have been causing an issue with the Ring Z-Wave network? Maybe a noisy device killing everything?
My Ring alarm sensors have been offline roughly ever since the ST problem started. My ST network started to work again today and all of the Ring alarms are now online without me doing anything to them. They have to be related somehow. Or it could be just like the articles you mentioned stated.
Good point!
It would be unusual, but possible.
Again, Z wave is intended for tiny messages sent infrequently, so usually there would be no problem. You can have an apartment building with a separate Z wave network in every home, for example, and it usually works fine. (One hotel did something similar five or six years ago.) i’ve had as many as five zwave networks in my own house without any issues.
But if one network were misbehaving, with constant chatter… Yeah, maybe.