It sounds like there is gunk underneath the key cap and into the key retainer clip assembly. If you're lucky, you can get away with removing the key cap and cleaning underneath it. Otherwise, you may need to purchase a replacement laptop key retainer clip.
If you need to replace a single key, here's how to do it: The easiest and most common method for removal is to place the index finder in the corner of the key cap and gently pull up. If the key does not separate from the retainer clip, try the same on an diagonal corner. When the key separates, there is a distinct "snap" sound as the plastic hooks on the underside separate. Once you are able to lift up on one corner, do the same to an adjacent corner. At this point the laptop key can be lifted up and removed from the keyboard base. The key cap will always have a little play in it so if this method does not work, a small screwdriver can be used to gently lift up the key enough so you can observe the way the retainer clip attaches to the back of the key. This information can be used to determine the best way to remove the key cap. For larger keys, there my be an extra metal bar that hooks into the base of the laptop keyboard. The metal bar will always slide into two metal brackets on the keyboard base so...
Hey I used this site a while ago when I had the same problem: http://www.laptop-keys.com. I ended up trashing the plastic clip and had to get a new one but I remember it only being $3-something. You may need to do that.
Try removing the key caps, cleaning underneath them and putting them back on. Chances are, there is crud underneath them and they are not making a connection. You can find some good tutorials on this at http://www.laptop-keys.com. You can also find replacement laptop keys there as well if you happen to break any in the process.