Entry Sensors - Add after removal

I am having the same issue. I can’t re add an entry sensor back on after removing. I put the system in Setup Mode and press the setup button the sensor but nothing happens. 

@frankis​ Suggest if you haven't tried the phone app, use that instead of the keypad. Much easier. If it doesn't work, possibly a battery or range issue.  If you have another entry sensor, remove it too and then swap locations. If one works and the other still won't, hardware issue and call SS to replace if you are an applicable monitoring plan or if still under warranty at nc.

Have you tried changing the battery with another entry sensor that has been added back to the system? Or if you have a spare button battery, swap it in and check that the sensor is powering on (should flash a blue light when paired and removed from its magnet). 

If it is lighting up, then use the keypad to pair if you were trying through the app, or vice versa. 
This is likely due to a power issue.