Veritas alarm goes off without time to get to the box?

Hello, I have a veritas 8 alarm that has been working fine. I always set it when we go to bed. Last week, when I came downstairs, the infra red caught me as usual, but then I used to have a minute or so to go to the box before the screecher went off. Instead, the PIR caught me, and then the screecher went off immediately. Since then, it has done this every time I've set it and triggered it from this PIR.

The other PIR downstairs is working fine- if I go out of the front door and then come back in,, it does the warning beep, giving me time to get to the box to put the code in before that awful screeching noise starts.

As it was working fine and it's not that old at all, I'm guessing I've knocked it or I'm doing something wrong somewhere. Can anybody help me put it back to how it was!? I have an instruction user manual but it doesn't seem to tell me what to do in this case.

Thank you very much!
Jo