Quoted from PinDeLaPin:
I've had my machine for 2 years and it's had an insane amount of plays and it's never one done that. What it does instead that drives me crazy is sometimes when I load up No way out with 2 balls it will sometimes go to pop the ball up the wireform and the ball won't make it out. It then falls back down but the ball behind it has already rolled into position on the popper cup. Now it sits there trying to eject 2 balls which it can't do and the game goes into ball search mode. Then you have to remove the glass, grab a screwdriver and wait for balls to shoot up the wireform and grab it with the screwdriver. At this point your multiball is shot to shit cause it will eject the other ball as well. Very frustrating i'm not sure if anyone else has experienced this issue. I've taken the whole assembly out and fully examined it but have not seen any reason for the problem to persist.
I had the same problem but I got it fixed. In my game the ball was hitting a small metal guide that’s about 1x2 inches big that is next to the cup that fires the ball upwards. I could actually see a small wear mark where it hit. Bending this out of the way a little made everything ok.