Alarm won't set?

Replaced the veritas 8 with new version, replacing wires identically. Completed walk test and bell test all ok.
But when replacing front and trying to set alarm.
The led lights for 5,6,7 &8 come on then 1 comes on and won't re-set.
Only have four PIR's,
Taken panel off again and wires are all in place and the 24hr light is flashing.
Any help please.?
Thanks