Door sensor tripping when door is closed!

My front door sensor trips randomly. It doesnt happen everytime. Worked perfct for a month but now i cant trust it. Nothing has been moved. When I arm the system the alarm will go off saying the front door has been opened. The front door hasnt moved though. Im not sure what to do. It doesnt do it everytime but often enough I no longer feel I can arm the ystem when I leave or when im home. Batteries are good. The space inbetween the sensor is under an inch and worked fine for the first month. Anyone else experince this? Any tips would be much appreciated! Thank you!