Joel's engine is unable to power his car the way it used to. The spark plug is working and the engine is compressing properly so what could be the problem

The problem could be a fuel related problem or a starting battery related problem according to the structure of an engine. An engine needs a good electric current, a good fuel flow, a good air compressing and a good cranking speed in order to start up. The problem on Joel's machine could be in its fuel pump or starting battery. It is because all of the engine parts working good besides those two parts.