If the Sensor light doesn’t flash when the door is opened and closed, the Sensor and magnet may be too far apart to register. Just reposition the Sensor or magnet. It’s a good idea to test the placement of your Sensor and magnet (see if the light flashes when you open and close the door) before you adhere each to the wall with the adhesive tape. As always, you can screw in all of the Sensors for your system using the supplied brackets.