Is this the Deere lawn tractor with the plastic transmission? If so, try cleaning the top of the transmission off. debris sometimes interferes with the linkage. They never expected it to work in the kind of debris in a normal yard. This sometimes works, sometimes it doesn't.
If its hydrostatic, make sure nothing is keeping your clutch pedal from coming back to the full engage position. Also, make sure your transmission is not twisting in the frame, this can slacken the drive belt and make it run slower.