The roto target should spin both during play and when the ball drains. Does it have the problem in both situations?

Looking at the schematic, it appears that 1/3 of the time the roto unit works differently. In any case, here's the circuit. The next step is to check the motor 3E switch.

For a switch to work 3 things are necessary:
1) When open, there should be a small space between the contact points (duh)
2) When closing, the long blade's contact point should push the short blade's contact point enough to move the short blade
3) The contact points should be clean, which they usually will be if #2 is happening

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