Start your search at the motor. Disconnect it from everything except the battery and make sure it works. Then build out piece by piece, testing each time you add something. You'll either find the problem and fix it - or discover that now the problem's gone!

Two heads are better than one. Ask a friend for help, and next time you'll help them. 

When you finish a step in your creation, take a final look. Make sure everything is working before you move on.

Replace batteries. Replace cables. Replace motors.

Make sure shafts are supported in two places.  Make sure the pieces supporting your shaft fit together tightly.

Make sure the gears are engaged. Add collars where needed to keep the shafts and gears from sliding.