baxi solo 2 pf boiler wont fire?

boiler wont fire unless clips on inner cover are released,
fan replaced 6 months ago, this week replaced pcb, still the same, yesterday replaced pressure switch,pipe to pressure switch ok, still not firing unless clips released after this occasionally refires without clips released, fan works and pilot lights ok, worked fine for 3 months after fan replacement then intermittent till last weekend, it is in a internal cupboard top floor with a vertical flue venting on roof ridge, house 8 yrs old not sure how old boiler is, any ideas please