Quoted from Betelgeuse:Typically, problems with the drop targets on SoF are related to the opto board under the mini playfield. These are very problematic and can cause all kinds of weird symptoms with the drops. An improved replacement board was released (I think by GPE?) some years ago. If you can't find anything physically wrong with the drops, this could be the source of the problem.
Sounds like his issue is mechanical. One drop target it dropping when it's neighboring target is struck.
This makes me think that either the "pull down" spring is too tight and not original, OR the spring that pushes the target flush to the assembly is missing and/or too weak OR the target rest bar is worn (more unlikely). There are TWO springs associated with each target.
OP, if you haven't yet, look at the manual. Taking apart the assembly you'll get the idea of how it works.