Alarm won't set?

The hard-wired alarm won't set - it just continues the slow beep indefinitely. It is showing Zone 1 as the problem which is the door - we had a similar problem years ago which was solved by an engineer replacing the magnetic door contact (recessed)so we bought a contact and replaced it but it has not resolved the problem. Could it be other wiring or are we missing something very obvious?

Bob
March 2016