Regarding small engines?

I had a bearing failure in the gear box of my Toro personal pace mower. I took out the ring gear and pinion. I replaced the whole gear box and axel shaft as it came as an assembly. Since I replaced it, I can't seem to get the mower to run or go in the grass. On pavement or concrete it goes fine but not on grass. I also replaced the belt. What possibly can be the problem? I noticed the new motors have a different style gear box. Does anyone have an answer to my problem?