You've got two main areas to consider - one is the coil circuit and the other the switch circuit that fires it. So far you've looked at the firing circuit. You didn't mention if the ball was fired out in ball search - if not, then it's the coil rather than the opto circuit.
You can go into the tests, and test the solenoids - put a ball in there if you like and when you get to the popper test, it should fire. If not, it's a coil problem rather than an opto problem. If it fires, you can eliminate the coil as the issue.
If it appears to be the coil, check the solder joints at the coil. The two wires from the coil (red and purple) go to a plug - separate and re-seat the plug.
If appears to be the optos, same for the four opto wires, check the solder joints, and then they go to a plug as well - separate and re-seat that too.
