Transmission problem.. I think

[TNracer]

I got a rc10b3 that i just has one wheel spinning faster than the other. The left wheel doesn't kick on until i have pulled the trigger about 1/5.

[B3 Bomb]

It sounds like your differential is loose. Take the diff out of the transmission and tighten the diff bolt untill the gear does not turn when the outdrives are locked with two allen wrences, or check to make sure there is a roll pin in the one dogbone/cvd axle.

[TRXboy]

Or the wheel that is not moving is to tight.

[Distro]

You could also jhave a jammd ball bearing or your cvd pin is rounded.

[atm92484]

If it has CVDs those could be the culprits along with the other things people mentioned. Check the one on the side of the car that is slipping and make sure the pin is in there and nothing else is broken.