Hi qmagnet. I am able to register and use the numpad keys properly, but when trying to SNEAK forward and leftward simultaneously, I do not move, or continue moving straight forward or straight leftward. However, I can still sneak fine in any of the other seven directions, and walk in all 8 directions. This happens as long as I have my movement keys set to keys that are not letters (A-Z), so this also occurs with the arrow keys, or any other number or symbol keys.
Thank you Kumasasa, this was the problem. I tested it in a few other programs, and my laptop does not detect when I press numpad 0 (sneak), numpad 1 (strafe left), and numpad 5 (walk forward) simultaneously.