Only one rear wheel providing power

I have a Craftsman lawnmower with a Honda engine. I replaced both wheels, bearings, and belt. I believe I put it all back together correctly but I'm not 100%.

Now the mower seems to be providing all the power to the left rear wheel and it pulls to the right when I activate the self-propelled drive. The rear right wheel only appears to be spinning slightly as seen when I raise the rear off the ground. The right wheel does not appear to be providing much power at all. What could be causing this?

I would think that one of the keys fell out and is not there anymore. I can not really tell because you did not give us the model number.

Blaz has it in a nutshell.
Only one axel that is solid so one wheel is slipping.
Depending upon model the drive pin can be missing, ratchet in back to front