A good place to start is refreshing your mesh network's connections via Network Repair. You can repair the network using either the SmartThings Classic app or the newer SmartThings app.
If using the newer SmartThings app, here's how to repair the network:
For Classic app users:
You can do this by first disabling the Health Check in your SmartThings Classic app and performing Network Repair. Here is how to disable Health Check:
And here's how to perform Network Repair:
Before sending any commands to your ZEN20 Power Strip, 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 device back into your network.
If Network Repair doesn't help, try excluding and re-including the ZEN20 Power Strip. It's possible that the communication route between SmartThings and the strip wasn't established correctly the first time. Exclusion might help the hub find the best route to the device.
Next, try installing the ZEN20 Power Strip closer to your hub. This is a good way to see if the issue really is range or if there's a better spot for your strip. Once you've moved the device, remember to exclude and re-include it to give them a chance to connect properly.
If you've tried all of the above and are still seeing the same issue, please don't hesitate to reach out!