A few thoughts come to mind.
You’ve probably done this already, but as A get ready step check each of the switches and make sure that the airgap is physically pushed all the way in if the device has an air gap switch.
1. Start with the Z wave repair utility
The first thing I would do is try the Zwave repair utility if SmartThings support didn’t have you do that already. This is pretty simple, it’s just an instruction from the hub to each individual device for the individual device to update its own neighbor tables. This will improve the general reliability of your Z wave network, because all the devices will know the most efficient way to pass messages along.
And the following thread will explain the error messages you might see:
2. Use the circuit breaker to cut and restore power to each problem switch
The next thing I would do is throw the circuit breaker once for any switch which is not responding. ( if two or more switches are on the same circuit, you only have to through the circuit breaker once. Just want to make sure that every problem Switch has the power cut and then restored.) The GE switches have some known issues where they can lose connection with the network. If you do a complete power cut which is what turning the circuit breaker off and then on again will do, this can sometimes restore their ability to hear the network. It’s another one of those “can’t hurt, might help” things.
If throwing the circuit breaker does bring any of the missing switches back online, then when you’re done doing all of them, run the Z wave repair utility one more time and that will make sure again that everybody is using the most efficient paths through the network.
3. Now start working on one switch at a time
After you’ve done those two steps, you can start addressing one switch at a time. Since these are Z wave devices, there’s an additional utility that can be a lot of help. That’s the “replace” utility. It’s normally used when you have a Z wave device break and you replace it with another of the exact same model. It allows you to do that without having to rebuild all of your routines and automations. The trick is that you can also use it to replace a balky device with itself, again without having to change any of your routines are automations. 
So it can save you a lot of work.
For this third step, you start by using the SmartThings mobile app to remove the Z wave device. And then you run the Z wave replace utility.
Here are the instructions for that:
I can’t guarantee that this will solve all the problems, but hopefully it will solve some without too much effort on your part.