When Arming: "Sensor Bypass"

On the phone app, when I press "Home" so as to arm the alarm, SOMETIMES it will say "Sensor Bypass" on the slider door and asks if I want to continue to rm it. When this happens, I slide the sensor off if it's base and slide it back on. Then I retry to arm the alarm and it works succesfully. The sensor is only 7 feet away fron the base so distance is not the issue. This has started happening within two days of setting up the system.