I think I just got this sorted out. The problem seems to be with the diverter fin hitting the back board steel guide before the plunger hits the coil stop. I bent the the diverter fin a bit near the shaft. Now it doesn't hit the steel guide anymore and seems to stay open the way it's supposed to. You can't realign the diverter just by loosening the small hex screw used to tighten the shaft since it has small recess for the screw to bite. If you try to do that the screw will just turn the diverter back into it's original position. That's why you have to bend it a bit. I took the diverter out and did the bending in a vice to get it from as close to the shaft as possible. I attached a picture for reference. The bend is very slight but noticeable. Hopefully this will be the fix for your games also.

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