If your SmartThings App is unable to discover a new device, chances are that you need to exclude the device before attempting to pair it with your App.
To exzlude a Z-Wave device in the SmartThings App:
Open your SmartThings App and Tap on the "Devices" Tab at the bottom of your screen
Tap on the Home Icon in the upper left of your screen
Tap on "All devices" near the bottom of the dropdown list
Find your SmartThings home Hub in the list of devices and tap on it.
Tap on the elispsis (3 dots) in the upper right hand of your App's screen.
Tap on "Z-Wave utilities"
Then tap on "Z-Wave exclusion"
Now follow the manufacture's instructions to delete the Z-Wave device from the current hub's network for from a previous hub's network.
Once your device is confirmed as successfully excluded,
then follow the manufacture's instructions to pair your new device with the SmartThings Hub.