Have you flushed it recently? If it is full of crud the safety systems may be shutting it off or there is so much gunk there is not enough water to heat.
There should be a valved that looks like a garden spicket on the bottom of the boiler. With the system off, open that (with a hose to a nearby drain) and let it flow until the water runs more or less clear. It may start out looking like crude oil!