Black Hole gets upset if it doesn't see all the balls in the trough. It will register the start button, but it just sits there waiting for all the balls to drain.

I would take the apron off and put three balls in the trough, then start a switch test. (Raise all the drops first so there are no false positives.) With three *balls* in the trough, you should see three closed fingers. (Sometimes the ball won't close a switch, but a finger will.) If any of those switches aren't working, the game just won't start. I have seen strange things happen in some troughs (my Firepower in particular) where the problem is really only evident when the playfield is in the game in its usual position, and less so when it's raised.

If only one switch is failing, it has to be something wrong with that switch or its diode. It must be a "local" problem.