It all depends on what you expect the mower to do and under what conditions. Is the grass dry enough so that the wheels show no moisture? Are cutting 1" or less of grass? Is the mower on full throttle? Did you look to see if the blades are on right, with both blades sharp edges facing the direction of rotation? Are you using the top drive speed? The drive speed has to match the demand placed on the mower - you may be going too fast for conditions.
It's possible the full-throttle engine speed is too slow - should be around 3200 RPM.

Just a few thoughts.