how to light my glowworm boiler?

I have had problems in the past and have changed the air pressure switch and printed cicuit board which cured my no heating problem my fault 6 months later is this.My heating has been making lots of noise for about a month which has now stopped .unfortunately now my problem is this in the morning when the heating should come on it dosnt unless you tap the front of the boiler to ignite it then it works ok.sometimes you have to do this a few times. can anyone help please.