Did you check to make sure there was nothing in the keyboard that is restricting that keys movement?
If there is nothing in it, then try the following...
1. Uninstall the keyboard driver.
2. Reboot the computer (without reinstalling the driver).
3. When it restarts, it should reinstall the driver on its own.
If this doesn't work, you may wish to try connecting/reconnecting an external keyboard and see if it works fine. If it does, then it could be the keys themselves that are the issue.
Also, if your device uses one, you may want to try updating the Synaptics Driver if the above doesn't help.