OK - I think the problem is located either in the door (coin switches) or in the coin stepper unit. I've seen similar on my Big Deal years ago, but that is a later pin 1977, and works differently.

I believe the coin switch has a set of contacts that is closed when the coin switch wire is in it's "rest" position. When a coin passes through, it opens this set of switches, and closes the middle blade, with another blade - this starts the game - regardless of the number of credits on the credit unit. If this second set of contacts REMAINS closed, a game will never start - the coin switch wire MUST return back to its resting position. IF, for some reason - the switch is not adjusted properly, then a game does not start correctly..

So the three blade switch should have one set closed at rest
When a coin passed through, the first set of blades opens and it closes a second set. The score motor should run and when the coin switch returns back, the ball should be served.

I think that the coin switch blades are not adjusted. The "disabled" set on the other coin chute worries me ! I should think that the "at rest" switches are permanently closed as described.