Another possibility could be that the diff needs some breaking in.
Mine came out of the box ULTRA stiff. I had to get on the throttle almost 1/2 squeezed before the motor got enough juice to finally spin the wheels!
Finally, after about 5 battery cycles is it now better broken in and responding with light trigger pull.
Also, LBRC posted a great checklist of things to check:
http://www.ripper7racing.com/r7rforu...read.php?t=575