When a SimpliSafe entrance sensor is not reacting, it typically means that there is a problem with the battery, communication, or sensor obstruction. Make sure it's within range of the base station, verify alignment, then change the batteries to fix it. For efficient troubleshooting and sensor status verification, you can also log in to your system via SimpliSafe.com. https://simplsecure.com/simplisafe-entry-sensor-not-responding/