Why cant i go full throttle on my rc18?

[blueb8llz]

can anyone figure why when i press full throttle, the truck wont move at all?
but when i go 30% throttle, then my truck moves. The funny thing is, if i pick up the truck so that the tires are in the air, full throttle works.
This is on a stock rc18mt with a Traxxas tq2 radio and reciever. and i changed the stock motor to a peak raider motor.