First, double check that you're using the SmartThings Classic app and not the newer SmartThings app. While it may seem counter-intuitive, SmartThings is still in the process of transitioning to the new app; in the meantime, it still isn't complete and doesn't allow for full functionality and programming of your devices.
We'd recommend disabling the Health Check in your SmartThings app and performing Network Repair. Here is how to disable Health Check:
1. Tap Menu (Android) / More (iOS)
2. Tap Device Health
3. Toggle the Device Health switch to the OFF state
4. Tap Done
And here's how to perform Network Repair:
1.Click on the Menu icon in your SmartThings hub.
2. Tap Hub is Online
3. Tap Z-Wave Utilities
4. Tap Repair Z-Wave Network
5. Tap Start Network Repair
Before sending any commands to your Power Switch, wait until you see the "Z-Wave Network Repair finished" message - this can take up to 15 minutes. Once repair is completed, try including the Power Switch back into your network.
If Network Repair doesn't help, try bringing the Power Switch closer to your SmartThings hub (you can also make use of the range test tool - here's how). Plug it into a wall receptacle near the hub, and then exclude it and re-include it. If the switch includes successfully and stays connected, it's possible the previous spot is out of SmartThings' range or there are too many interferences in that location.
We'd also recommend excluding and re-including the Power Switch from the installation location directly. Similar to the idea behind Network Repair, it's possible that SmartThings hasn't found the most efficient communication route to the switch. This can occasionally happen when you include a device near the hub and then move it away to be installed.
If you've tried all of the above and are still seeing the same issue, please don't hesitate to reach out!