Can you clarify? If the pilot isn't on it sounds like the thermocouple is not getting hot enough for the boiler to stay lit. The thermocouple it a thermostat that sits in the middle of the flame of the pilot light & has to be at a certain temp for the boiler to fire up.
If you mean the pilot is lit & the boiler tries to fire up the main burner though won't, it could be a vntilation issue. Check your outside pipe is not blocked? Else get a professional in cos the gas bits shouldn'd be messed with. Might be something to do
Lee
May 2006