I thought I'd share my experience with the Metallica drop targets. Initially when I got the game they seemed to work great for perhaps 50 games, and then targets 2 and 3 (the back 2) would only sporadically drop, sometimes staying up with a dead-on hit.

I received a rebuild kit from Stern which contains targets and stiffer springs, but after taking it apart and putting it back together I noticed there are 3 more thin washers on my original than the instructions account for. I followed the instructions and the targets behaved the same, with the second two still sticking. One thing I noticed was the lip of the drop target was hanging over the edge of the metal more on the second two than the first, so I decided to use 4 washers on the two sticky targets to have less target sitting on the lip, which should allow it to drop easier. It did help, but I also noticed the stronger springs were holding it tighter to the lip than the original red springs, so I reverted to the softer springs, and that helps. But it still wasn't 100%.

Examining the difference between the first target and the second two I notice one other difference... the spring on the first target was completely clear, and on the second two the spring rubs the target. This is illustrated in the following pictures. The fix was to make a cut in the steel to mount the spring away from the target, effectively matching the way the first target was done.

target_original_spring.jpg
target_spring_touching.jpg
target_spring_after_mod.jpg

The rebuild kit did not help me. Some people have indicated switching to the stiffer springs did the trick (those are included in the goodie bag), but it didn't help me either. ACDC drops are side by side, so the mech is different enough, but the considering my solution involved tweaking target spacers and spring location that may also apply to the ACDC drops.