If you have an EM pin that always stays on the same ball, there are several
things to check. On single player games check the ball count unit. Make
sure the rivets are clean (and the wipers are contacting them), the
step-up/down coil plungers are not seized, and the spring is not
broken. Manually operate the stepper to check for smooth operation. On
multiplayer EMs, check the player units for the same things. Also check the
ball return/outhole switch, which tells the game the ball drained. Another
source of this problem is the ball release trough switch. I recently picked
up two Gottlieb multi-player pins which both wouldn't move off ball one!
Cleaning and adjusting this switch did the trick. Try the above before
moving onto relays, etc.