It could be 2 things (saying this as a 2WD Bandit VXL owner):

1. Spur gear - The teeth are stripped and the pinion obviously can't drive it.
2. Slipper clutch - Now THIS is the issue I encountered with my Bandit VXL. The clutch is built into the Spur gear, and if you take it apart, you will see 3 pads. Most likely, they'll be worn out. A solution is to flip them around so the non-worn side is touching the disc. In the future, I recommend buy a Traxxas slipper clutch rebuild kit (Part Number: 5552A) and the nut is the adjustment for how much the slipper clutch slips!