I have a WMS Aztec that I have done a full restoration on. I have been having problems with the ball count stepper unit not completely firing all of the time. I have completely taken apart the stepper unit and cleaned it, replaced with new springs and replace the A23-600 coil with a NOS from pbr.
What I observe when the stepper fails is the step up coil does not fire completely, as if it is under powered. I basically go through 7-8 balls in order to finish a 5 ball game.
Is it possible that the coil is not always getting enough power to fire? When it does fire correctly, all is fine. Had the same issue before I replaced everything.

Torsion spring is good.

Thanks Ken, I'll go over the switches.