I was going to suggest making sure that the buttons were grounded properly. Sounds like you figured it out though. You should see what some people do to their machines and then wonder why they don't work. I found 1 machine where they had looped all the grounds together, no actual ground mind you, just a big loop and guess what? None of the controls would work! Huh? Amazing...
Pessimist: Oh, this can't get any worse!
Optimist: Yes, it can!