Ive taken the top off and its not turning under power ive checked the motor leads and there ok so i think i needs a motor

I have a DDA40x  and the rear motor is not working it just drags the rear wheels what can I do to fix it

Yes it is a sound unit it is new just out of the box. It runs fine in dc I think I need more power for my nce power cab

how do I fix the skipping in my DDA40X it happens when I try to go faster then it starts to skip in sound when that happens it stops moving and sits there skipping or ticking