Quoted from lyonsden:
My bet is a mechanical issue with one of the switches. I seriously doubt it is the ROM, especially if everything had been working correctly. Make sure to check the switches in the subway and trough using a ball (not your fingers) and double check to see if any of the balls appear to be magnetized.
with the ball not your hand like this guy said. had this same exact issue.
It's kind of tricky to do in the subway, because of how it works. anytime you flip the playfield the ball want to move obviously
just readjust the switches, making them a little more sensitive.
come to think of it I've had this happen on 2 separate occasions. the main switch at the entrance needs to be triggered then one of the three switches in the subway. the first time the first switch worked most of the time then others times it didn't, the second it was switch 2 or 3 in the subway