Are you sure you put the pins in the axles when you reassembled? If you tightened the nut on the spur gear(slipper clutch tension adjustment), and that didn't help, then you need to look at the next thing. If it's slipping excessively then you should be able to see where it is. Leave the body off and put it on the ground in front of something(like your foot) and give it some gas.
You should be able to see where the power is lost.