My Baxi 2PF boiler only fires on to the burner when I unclip the back cover

[rodgerd2005]

have recently restored a house that was lying empty for over 3 years and have found that when I turn on the Baxi 2 PF boiler it only puts on the pilot light and clicks away without igniting the main burner. When I remove the white cover then undo the clips briefly on the gas tight panel behind , it always bursts into life. The boiler will them run perfectly all day. If I turn the boiler off and on throughout the day it is usually fine but if I leave it to go cold I have to unclip the cover again. Can anybody advise me how to fix this problem please?