What model is it?
Depending on the model you have, you may need move a pin to lock both wheels. Look at the wheel on the right side. There should be a pin and ring. If the pin goes through the axle only, that wheel isn't locked, so only the left wheel is driving. If that's the case, remove the pin, lift that wheel off the ground, pull the wheel out and rotate it so the holes line up and put the pin back in. It should now go through both the wheel and the axle, so that both wheels drive.
If you don't see a pin and ring, you may have a differential. If that's the case, someone else may be able to help you.
