Ariston EuroCombi Odd Problem?

Everything was working relatively normally until the fan stopped working so I took it out and revived it.
I reinstalled the fan and tested the boiler. The burners fired as normal and everything seemed good..
Until I put the inside cover back at which point it went out and wouldn't relight (Fan still working as normal) Lifted the cover off and the burners lit. Did this several times to prove that I wasn't going mad! It works with the cover loosley fitted so that air can get in but as soon as it's tightened down the burners go off and won't reignite.
I'm wondering if the fan is still not working at full efficiency? Would that make sense? Or am I missing something else?