Likely it's a problem with the payout lock lever. This is the "fifth click" on a five click machine. It's a bar that resets the payout slides and holds them until after the vertical arms release to penetrate the disks (fourth click) and push the horizontal arms clear of the slides, then it withdraws (fifth click) and lets the slides pull back and drop coins. If it's maladjusted one of two things can prevent payout. Either the slides are resting on the ends of the horizontal arms, so that the vertical arms can't push them against the friction, or else it doesn't release at the end of play and let the slides come back. The Jennings manual says "The payout lock lever at the point where it rests against the payout slides should be GREASED frequently to prevent binding" (capitals in the original).