If this happens to be one of the 2002ish models of Clavinova (like my CLP-970), there is a known problem with the keyboards in some of these models. I had an issue with sticky keys in 2009, and Yamaha replaced the keyboard at no cost to me. I called Yamaha USA tech support, they verified the model number and serial number, and sent the keyboard to an authorized repair facility which came out and replaced it.
Apparently, this issue is either white keys being too thin at the pivot point or the lubricant used at the pivot deteriorated the plastic. Here is a link to a UK site with information on this known issue. Here is a link to a thread on the CVP forum archive regarding this issue.
I hope this helps some.