Hello My name is Ivor I hope that I can give you a couple of things to try .
As you have changed the Alarm panel, I would suggest you re-look at the detector heads. If there are batteries in them these are used as a back up and may be low in power..The other thing to check is if there is any dirt or dust in the heads or the contacts are dirty, check the head connections. If these are ok another is to check the earth continuity as there may be a loose connection.
I hope that you can solve the problem with my suggestions.