the problem is probably oxidation. the rubber keys each have a small
black spot of conductive rubber. clean these vigorously with a q-tip and
isoprophyl alcohol (rough them up good to remove the oxydation from the
rubber). next use an ink pen eraser on the little copper foil pads that
correspond to each key, but not so hard as to damage or lift the foil
trace! clean all eraser residue with a q-tip and alcohol and you are all
set! good as new!