I agree that the 2 sensors have not been working.
They were connected in parallel, I have tried each of them separately connected to zone 4 and they both work.
Is there a way of connecting the unused PIR into the system?
If its complex, disruptive and/or expensive I may decide to leave one of them permanently disconnected. The house is fairly well covered and it would still be better than it has been for the last 20 years.