Ok, over time keyboards get dirt, hair, and all types of junk built up between the keys, and underneath the keyboard. Cleaning isn't hard, but it is a small project.
You will need:
Can of compressed air (from the nearest electronics store)
Tweezers, needle nose pliers, or something to grab debris if you cannot reach it with your fingers
(possibly) Rubbing alcohol and a cotton swab
Unplug the keyboard and let it sit for about 10 minutes to drain the residual power. Remove the legs from the piano, and turn it upside down. Remove the screws from the bottom of the piano. All of them. The cover will now come off, and you'll be able to see underneath the keys. Use the compressed air to blow out debris, and inspect the keys.
Some of these models have weighted keys. Take a look for broken pieces on the problem keys while you have it apart. If you have any broken keys, let me know and I'll connect you with a new key.
Be sure to remove all of the debris. Take your time, the more time you spend on it, the better it will feel.
IMPORTANT: Do the work when you're not standing on carpet, or try not to shuffle your feet. Static electricity can damage the keyboard. Don't touch any of the circuits. If you have to touch circuits, touch a screw on a metal appliance to drain the static from your body before touching the electronics.
Let me know if you run into trouble. Half of the time this is caused by debris, and the other half is due to a broken key. I await your reply.