Hey there. I had a few problems with that too (when I was new). You may want to double-check to make sure that your pinion gear and motor screws are tight (use a box-end, not ball-end allen). If this does not work, check to ensure that your slipper and transmission are set up properly. (BTW, I'm new here, not new to R/C)
PB: Shiftin' gears and kickin' rears at SoCal since 2005.