Quoted from cjmiller:

I think the biggest trap people fall into with schematics is over-thinking it. We all tend to build a model in our heads to explain what we think should be happening (rollover closes switch, switch trips relay, score motor starts . . .) until it becomes overwhelming. That's when we start adjusting switches based on hunches and educated guesses, and usually cause more problems than it fixes.
The best repairman I ever knew was an old school operator, and he used the schematics a bit differently than most. His main tool was an 8 foot jumper wire with alligator clips at both ends.
He used the schematic, but he would jumper from whatever wasn't working to the first switch shown on the schematic. If that didn't make it work he would move on to the second switch. Eventually he would arrive at the problem, and he could find and fix things faster than anyone I've ever seen. He didn't give a second's thought to the theory involved, the schematic was just a map that showed where to put the alligator clips.
I just went through this last week. I wasted hours because I was convinced that I knew what the problem was. When I finally gave up, dug out the jumper wire, and started following one line on the schematic, I found the problem in 20 minutes.