Lawnmower drive works on pavement not grass

My self propelled Murray (about 9 years old) stopped propelling. I changed the belt and cleaned the pinion gears (one was really gunked up with grass & dirt). The belt is set to be the tightest setting according to the holes on the idler arm. The mower now will move forward on it's own, on pavement but still must be pushed on the grass. When I push down on the back wheels to lift the front wheels off the ground, the right wheel spins fast as does the shaft. The left wheel (which is by the belt) turns, but slowly. What could be the problem? JB in CT
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