sorry I know this is super old. but Just wanted to throw some comments at it since I stumbled upon it.
The metal guide that the ball follows when being shot up from the up kicker coil in the trough to the shooter lane, the welds crack and can easily lead to balls not entering the shooter lane, or bouncing back in. Check those, and you can also check if the coil sleeve is old/clean, to make sure the plunger is free to move. Otherwise if there is a power issue, do any other coils have weak power? Check power at the coil lugs.