If you wired iotty Light Switch correctly and you screwed the device into the wall but the touch works only without the faceplate, the problem could be that the screws are too deep in the wall, this deforms the device causing the faceplate to not connect correctly to the touch surface.
To solve this problem loose up the screws enough to unbend the device.
Then after you attached the faceplate restart the device from the device setting on the app in order to recalibrate the touch.