Some people have reported this in the past with the older GE devices, particularly the ones without a neutral. Seems to require turning off at the breaker box and back on. No idea why it happens. Search the forums and you should find some discussion. It’s not a SmartThings issue, btw, Vera users report the same thing. Sometimes strengthening the mesh by adding more devices helps, which would make sense if it had always been unreliable, but why should that occur long after initial installation? Weird.