boilerwill not fire up?

have a volkeri boiler it will not light up unless i open cover plate to burners have changed fan and pressure switch. it will carry on running when you close plate but will not relight with it is re closed it seems to be somethink to do with flue but is all clear they have built a new exstention next door so flue is now between 2 buildings any thoughts

steve
April 2013