Have you tested the opto switches for those drop targets? They should be in the switch test. It sounds like they either need to be cleaned or the optos themselves need to have their connections reflowed.

To test in switch test is easy, go to the switch edge test, I think. Manually reset your drop targets. Then drop them individually in the switch test to make sure they register.

If you want to test your switch matrix, you can do that at the CPU board, you can also do a switch test, it will show you any shorted switches. By isolating the PF/switches and testing on the CPU you can test the matrix itself. I would first do the edge test and verify your optos and connections on the opto board are ok.

When you look at your drop target assembly, you'll see a small circuit boad - that will have your 3 opto switches and the 2 connectors which go back to the CPU board. You'll see the colored wires will correspond to the row/column for those switches in your manual. I think on your machine that opto board is held in with 4 C clips, there should be some rubber grommets which help isolate the board from vibrations when the targets get hit. If those have hardened or cracked it might be a good idea to replace them with new ones.