normally yamaha pianos has this sort of problem. the broken butt cord. after more than 20 or 30 years
the hammer butt cord will break off by it self. when you look closely, the cord will become ashes.
when butt cord broken, it causes unhook with the hammer butt spring. when this situation occur, it will affect the touch and repetition of the piano keys. the piano hammers may take time to come back whenever it strike the string, or it might come back half way and stuck in the middle of somewhere which is very irritating.
however, butt cord can be replace. they are more ways of understanding why your piano keys are not functioning, this is only one of the problem, you can find and read more here http://www.blogger.com/blogger.g?blogID=1514498791870230169#editor/target=post;postID=8364125876403800796