magnet won't close zone?

I removed a window so as to install an air conditioner in it's place. I've tried to install a magnet to replace the one that was on the window. I lined it up precisely with the sensor that remained still on the window frame. The zone that covers that window won't close and show ready to arm. I tried all different positions for the magnet, reversing it in case the poles of the magnet were positioned backwards. Nothing I do will cause the zone to close so that I can arm the system. Any suggestions? Thank you.