GE zwave switches have an unusually high failure rate, typically about six months after they come out of warranty. Maybe as high as 15%. Nothing to do with SmartThings, you’ll find the same issue discussed on many other home automation boards.
The switches are technically not defective since they are out of warranty, but it can be annoying. They are cheap enough that a lot of people don’t care if they have to replace 10 or 15% of the switches as they still saved money on the overall project.