The pilot light will only light when there's a call for heat - hot water for the taps or for the central heating.
Before the pilot light lights the fan runs - an air pressure switch makes sure the fan is running properly - the gas valve for the pilot flame is opened and sparks are made to light it.
Call for heat - fan runs - pilot gas valve opens.
No call for heat = no pilot flame
You do not say the make or model of your boiler, July 2015
