You're using drills yeah? Sounds like the ring gear inside the gearbox is rotating/slipping and not transferring the torque (turning force) to your wheels. If you've used grub screws in your drill modification, tighten them up and see if that helps. If you've used the O-ring method, ditch it in favour of grub screws.

Can you upload an image of your setup? A picture paints a thousand words as they say.