Front door always in open mode

I've just purchased and installed a Ring system; 2 contact sensors for front and back doors and 2 motion detectors for a couple of rooms. I tested them all out and all worked apart from the front door, which is for some reason now stuck in 'open' mode which means I have to bypass it each time I arm the system. Yes, the magnet on the door is not fully flush with the main section on the frame but this is also the case with the back door, and that works fine. Please help!