If the wheels are not touching the ground, use bigger wheels or move the motors and axles down.

The wheels could be going too fast for their own good. They spin too fast to be able to catch the surface. One way to fix this is to use larger wheels or to gear down the wheels (i.e. use gears so that you end up having the wheels turn slower than the motor). One could also lower the power levels of the motors through program. Sometimes, the type of surface that the wheels are driven on can affect how well they perform.

ses