Actually, your trough switches should be 15, 16, and 17. Switch 26 is the proximity sensor and has nothing to do with loading balls. It's only purpose is to sense(or not sense, technically) the ceramic powerball. That's a simple logic operation, too. If switch 15 is closed and no signal is sent from 26, it knows the ball is ceramic and not magnetic. Also, if the ball can sit in the shooter lane with no ball search, that is a good thing. It knows where the ball is because the switch there is working properly.

What I'd do is remove the apron and go into switch test mode. Load a ball into the outhole and while watching the screen, make sure that at as you manually slide the ball to the right along the trough, every switch the ball touches fires off. There should be a visible notation of it on the screen in the switch matrix graphic on the left along with a description of the last switch that just triggered on the right side of the screen as well as an audible tone denoting that a switch was triggered.

Why you want to use a ball to test it instead of just your finger is because your finger can actuate the arm of a switch much further than the ball can and you might have been getting a "false positive" on a switch, namely 15, 16, and 17, the trough switches. If the ball doesn't trigger a particular switch, but you can do it by hand, a slight adjustment to the switch's arm might be all that's necessary to get it to register when a ball is in that place.

It could also be that the switch itself is just flaky. I had this happen on my TAF's ramp exit switch. All I had to do was to cycle the switch over and over to "clean" it up inside. Sometimes those switches get "sticky" and will just stop working. A little bit of actuating them over and over(like 50 times or so) frees them back up and they usually start responding to the light stuff again.

Try this stuff out and let us know what happens!