Yes, they can fail, they have a non return valve inside which can break by either blocking or falling to bits, I have replaced many in my time. for replacing, expect a bill of about £100.
The way to check is to have someone at the boiler, reseting the lock out and someone looking at the tigerloop to see if fuel is present and if so, is it moving about? If no fuel is present, and the oil level in the tank is above the tigerloop, its either screwed or the oil filters blocked. If you cant see inside the tigerloop and UV rays attack the plastic, i'd have a new one anyway as its a matter of time until it cracks and leaks.
Mr Boiler
July 2008