I've just finished 9398 4x4 Crawler. The rear motor works properly, but the front motor doesn't: The wheels seem to be locked, although they spin freely when I turn them by hand. What could be the problem?
Check if Cross Block 90° that is placed in step 53 of instruction book 1 is in the correct orientation with the round hole placed over the motor axle.
Mounting it the other way will prevent rotation of the motor.