First check if all the mechanics are able to move smoothly, most of the time a motor skips due to mechanical build errors. Use some lubrication on all the bearings and smooth rods. If this does not help you can use the trimmers on the motor drivers (see the manual on how to do this) to give the motor in question a bit more current (and thus more power). Do this in 0.05V increments. Do not exaggerate with this setting!
Minimum: 0.4V Maximum: 0.65V