(solved!)
FIVE hours later, both drop target assemblies have been cleaned and installed with new targets. Everything works overall. Targets were working fine before the clean/reassembly.
Here's the problem. It's a Bally target assembly adjustment issue I gather. When the machine resets the targets they sometimes (1/5 times roughly, and it can be any of the four targets) go too far up vertically, and the little metal levers come out of the slots. Once this happens, a ball strike will not drop them. I have to use may hand to wiggle it back to where it belongs.
I see that there's a horizontal bracket at the very bottom of the target assembly that adjusts with 4 hex screws (1/4" play in it at least) I am wondering what this adjustment is for? I've tried it in a few positions to see if it fixes it. No dice yet.
Fact review:
1) O-R-B-S (any of the four) can get loaded up to high vertically
2) It happens 1/5 times roughly
3) It was working fine before the cleaning and install of new targets
4) all the switches work (not part of the issue)