Keys are sticking on keyboard HP Pavilion DV 9000

I have an HP Pavilion DV-9000. the keys on the keyboard are starting to stick. It started with the letter C, then X, then S, now it is sticking on the number two. Any idea what is happening or how to fix it? I keep popping off the key covers and cleaning under them, but I see nothing. right now I cannot get the number two to type at all. I might add that when I take the key cover off and press on the rubber diaphram underneath, the key will work. Thanks.