Wrb might be on the right track with the spring suggestion. I would be careful to put something under the spring so that it doesn't chafe the solenoid and eventually short it out.

That spring's function is to keep the target pulled forward and on the half-moon shaped "ledge" when the target is up. Your picture looks like you have different springs than those on my EBC.

Also, it looks a little bit like the spring on the drop is touching the edge of your solenoid. If that is the case, that is working against you. You might have a little bit of wiggle in your solenoid bracket mounts to rotate it counter-clockwise and take that out. My spring in the front (pictured) drop target just barely clears the lip on the solenoid and does not "bind up" against the drop target when the target is up (you can see our back targets look similar when down but the front targets look different when up). I wonder if someone substituted a slightly different size of spring in your assembly and the geometry difference is causing the problem. I know it is hard to find parts for EBC since nobody cross-references the goofy Bally Midway part numbers in the manual. The solenoid and flipper assemblies seem to be fairly unique.

I wonder if you could experiment by taking out the spring and using a section of rubber band in its place to see if using a smaller diameter spring (along with more or less tension) fixes it.

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Your drop should look like the picture, and it looks like the drop that is down in the background has the correct shape. There is only a small half-circle where the drop is flat and holds it in the up position. I don't know if I'd go too crazy with a Dremel there unless the half circle is not quite squared off. It only gets smaller with filing or wear.

I like the idea of maybe shaving a little flat spot in your solenoid to allow the spring to clear. There is maybe 1/8" of "takeup" in the target and your spring might be taking half of that up because of interference and the spring is surely vibrating all over the place for a moment after your solenoid forces it up. The bouncing spring might explain how the target locks in the up position manually but doesn't reset by the solenoid.

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