Check the switch in the plunger lane, and make sure balls that get fired into the trough aren't bouncing back into the drain. My Stargate was occasionally losing track of balls because the one-way gate in the trough was in the wrong position and balls would sometimes roll back and hit the drain switch after being fired by the drain kicker.

I've included a pic of my trough for reference. Your coil doesn't appear to be correct like pinmike suggests, however, I'm also wondering if for some reason the ball return is not adjusted properly and isn't sticking out far enough. Mine takes quite a bit of force to get a ball past it, I can't imagine a solenoid being able to hit the balls that hard.
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