Check the voltage in the individual cells, check the settings on your radio. Also check that your rear diff is not slipping, and check the tightness of the rear wheels.
The last race I entered the rear nut came loose on one of the wheels, and it was just enough for the rim to get away from the axle pin. The pin spun and stripped out the back of the rim, so when I gave it throttle the axle spun in the rim causing the tire not to turn.
Though the later can be a rare occasion, its worth looking at.
Original RC 10T, T4, RC8Te, Tekin RX8. Losi Ten SCTE, Slash 4x4.