Please re-check the Sensor UID you input to ensure that you inputted it correctly
The Sensor may not have power. Please ensure the batteries are fresh, and that the battery tabs for each battery are firmly cradling that battery. (You may need to replace the batteries on the Sensor, or bend a battery contact.)
If the Sensor is currently un-registered, you will see a Red LED blinking on the front of the Sensor every nine seconds. If you don't, then the Sensor could already be registered, recently unregistered, or doesn't have power.
Was the Sensor recently unregistered? It takes about 40 seconds for the Sensor to un-sync from the Relay so that it can be re-added. Try waiting a little and adding it again; wait until you see a Red LED blinking every nine seconds.
The Sensor could be out of range of the Relay Station. Generally, Sensors will operate well within 300 feet of the Relay, but environmental conditions can affect the range of communication between the Sensor and Relay. Try moving the Sensor closer to the Relay Station.
You may need to logout and log back into the mobile app as the authorization tokens may have expired. You should also update to the latest version of the mobile app.