Poterton Boiler needs a bang to get it going !?

We have a faulty Poterton boiler (60F LPG), that has an intermittent fault. When we switch on boiler, it sometimes fails to ignite the main flame (pilot ok). The weird thing is when we tap or bang the side of the boiler it always then ignites. We have swapped out circuit boards, checked contacts etc but still the same fault.

Any ideas