Help CenterPlease check the following.
1. Check if the red led light on the controller turn on when you press a button on the controller.
2. Select the sensor icon on the alarm panel ((o)), then select Controller
3. Delete the controller that is already paired and then try to pair the sensor that would not pair.
4. Also ensure that non of the PIR motion sensors are activated (no one walks by it) while trying to pair the controller since the PIR is always active, it can interfere with the pairing.
5. Check if the second sensor that at first would not pair, pairs with the alarm panel, then try to pair the sensor that you deleted.