I had a similar issue with a similar game recently. Make sure the unit that counts the WOW extra balls decrements fully, and the switches on the back side of that unit open or close when in "home" or off "home" position (controlled by pin mounted on stepper unit ratchet gear).
The regular 1-5 ball count unit registers "5" (it's an add-a-ball, so it counts down) only after the outhole kicker sends the ball over the ramp switch under the card tray.
My "Lucky Hand" was doing the same thing, and it turned out to be misadjusted switches on the upper (WOW) unit.