When the boiler is asked to fire up (call for heat) first the fan runs then a pressure switch senses that the fan's running OK then the pilot light comes on and then a sensor tells the control board that the pilot flame is good and hot and then the main gas valve is opened.
You do not say if any of these things are happening with your boiler.
My old Potterton had a habit like the one you describe - sometimes well behaved and other times not - it used to stall with the pilot flame burning and just sit there refusing to go any further.
Grit had got into the pilot injector - the hole was partially blocked and the flame not hot enough to trigger the sensor - a specialist engineer cleaned it out for me and restored the flame to full strength - problem solved.
You would do well to call in an expert but be warned - a few Gas Safe engineers change things willy-nilly until they find the cause of the problem - and you pay.
Ask the engineer when you call him if he does proper troubleshooting - analytical diagnosis - or if he just messes around changing things on spec.
Do not be afraid to challenge him and say, "What do you mean by - it's gone ? - what does gone mean? How do you know it's gone? - Show me."
Good luck...
Peccavi
June 2013