How can I fix my Vaillant 242 Boiler?

I have a Vaillant 242 Boiler that's been playing up lately. I've had a company come and look at it, they've sent 3 different engineers and 5 occasions and attempted to replace 3 parts with no luck.

I haven't been happy with how the repair company have dealt with it so last night I had a little look myself. I worked out that if the boiler is turned on with the front of the boiler taken off, it ignites every time, but as soon as the front is put back on, it fails to ignite.

I'm not technical so apologies if this isn't quite worded right but basically, we could get neither heating or hot water. Turning the boiler on would start the gas and the electric spark but the spark wouldn't ignite. It works perfectly, without fail when I take the front boiler plate off though.

Does anyone know what this could be? Sounds kind of strange to me and I don't know how it wasn't picked up by the engineers.