Open again, check for broken/corroded traces from that button.
Clean BOTH the contact pads on the buttons themselves and the PCB. Isopropyl will do the trick, any concentration. If the brass contacts on the PCB look corroded, then I would suggest something called TarnX. I've been using it for a while to restore water damaged portables with a LOT of success. Just make absolutely sure to clean it all off when you're done. If left on metal, it will eventually tarnish/corrode the metal part (ironic, as it's used to remove tarnish).