One way to find this is as follows......turn off all of the cars, power up one car and see where it goes. It should go to the floor with the stuck button. The only problem is this, getting the owner to let you do it and keeping the public from getting in the way. I had a duplex doing the same thing. 12 floors of a parking garage. By doing this method I was able to find 6 buttons that had one of the contacts defective. The innovation buttons have a little spring inside of the black plastic contact that will break and occasionally make the contact but not roll off of it. Another way it to go to each hall fixture and physically checking each button.
