I've heard of a fix for button pads that were too worn or dirty, glue a piece of tin foil to the bottom of the pad where the carbon coating is/was. I've never done this so I have no idea if it would work, but it does make some sense and seems like it would fix an otherwise non working pad. I've fixed plenty just by scraping it with a knife and roughing it up a bit as well as cleaning the board contact area.