ferroli modena wont stay alight?

im working on a modena and the problem i have is the boiler wont stay alight when the front chamber plate is fully in place and sealed.

As soon as i remove the cover and let a little air into the appliance the burner will fire up and stay alight for a long period of time.

Ive changed the air pressure switch hoping that would solve the problem but no such luck.

any other advice would be much appreciated

cheers