I’m sure that’s very frustrating. 
There are two separate answers to this.
The first is just that smartthings does have a problem with their device health reporting. They’re supposed to be working on it, and you can hang in and see if it gets better. But there’s not a lot that we as customers can do about that other than turn off device health reporting.
https://support.smartthings.com/hc/en-us/articles/214529303-Device-Health
The second is to do the same kind of troubleshooting that a field tech would and just start trying to improve your network backbone. However, this takes a lot of time and trial and error effort, and it can’t do anything about the first answer, so at best you get to situation where you know it’s not you. I’m not saying you should do that, but it is an option if you want to put some time in on it.
If you do want to do the second option we need to know a lot more specifics. In particular, we need to know the exact model number of the motion sensors, and we need to know what zigbee repeaters You have and where they are located.
You’ve probably done all this already, but if you are interested in doing more troubleshooting, start by reading post 11 in the following thread, then go up to the top of that thread and read the whole thing, then manually map out your zigbee backbone, make any improvements in location that jumped out at you, do a zigbee heal, and then let’s see where we are.
Again, I’m not saying you should do all that, I’m just saying that if you want to, there will be lots of community members who will be glad to help with suggestions as you go down the “strength the mesh“ path.