The problem is most likely the x-over track not being perfectly flat. This makes it difficult for the LC extension in the guide from consistently hitting the lower cut out in the flipper? if that's not it, is the outer lane flipper feeling tight? If so, you'll need to loosen the screws under the LC to free up the flipper movement.
If the problem occurs only on some cars, then the issue is a slight warpage on the chassis of those cars raising their front end height and thus raising the guide so the LC extension will not hit the flipper. In addition, it could even be your braids being too springy which also raises the car height (so flatten the braids).
No moving parts? The SCXD I'm familiar with uses the drop pin in the car to catch a low trigger of sorts, which moves the flipper to the desired position. After the car passes though, it returns to the original position. While it's not electronic, it certainly has moving parts that can be restricted by deformed tracks.
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