Without being able to see it very good, I am suspecting that the ball lifter got tweeked a little bit while folding the head down at some point. Did he move the game from one place to another and fold the head down? That lifter is vulnerable to damage if it isn't moved up a ways on the mech before folding.

So the repair was probably done many years ago, when the game was on route or just before your father bought it. It held up until now, and it is letting go now and causing the problem. That is a piece that needs to be reproduced, but to the best of my knowledge, it has not been or at least isn't available now. You will have to do what you can to repair it again. Hopefully someday they are reproduced and you will be able to install a new one. I would buy one for sure, even though mine is not broken.