Quoted from ibjames:
When I do a trough test, all optos work except the elevated one that is towards the shooter lane. My question is: Has that ever worked?
Yeah, it is an important opto. It is obviously causing the failure you are seeing.
I suggest replacing all the transmitter optos on the trough transmitter board. Transmitter optos are on all the time when the game is on and after 20 years, they all will die at some point. Also you would want to check all the other solder joints on the board as well for any cracked solder joints.
search for p/n QED123