In my experience, this has often been due to dirt or some liquid. Do Not use contact cleaner near any of the rubber parts! However do gently wash the circuit boards and examine the connections. Once on a Casio, I knew coffee got into the volume knob area, and it stopped working. ON a hunch, Casio sent me a replacement part (well, a little group of parts), and it fixed the problem. I had to replace the little PCB that housed the volume stuff. It was a $600 keyboard so it was worth it. Look for sticky ness or water damage and then contact Casio!
