I had this problem with my keyboard. Number pad was working fine. Tried everything... new batteries, reboot, reinstall drivers, check for stuck keys, etc. Nothing worked. I opened the battery compartment and took out the batteries. Turns out that one of the -ve battery contacts inside the compartment was tarnished. (The -ve contacts are the spring-loaded ones at the wider end of the compartment). It was blackish when it really should be silver. I used a small piece of fine sandpaper to gently remove the corrosion and then replaced the batteries. Keyboard worked fine after that.
This was my issue. My old battery also corroded and made one of the "-" negative terminal brownish and covered with some hard materials. Polish it with sandpaper or clean with a little bit baking soda on paper, solve it!!
Thanks!