That's a very easy fix,pull the rear wheels off and remove the c-clip that holds the pinion gear on,remove the pinion gear and there will be a keyway with a small spring under it,clean the keyway and the slot it rides in with brake cleaner or the best way you have to clean it,then put a few drops of oil in the slot and make sure the keyway works freely in the slot it rides in,note,make sure you put the pinion gear back on the same direction as it come off.....what happens is that the keyway slot gets dirt ect,ect in it....I own a HRR,HRX AND A HRC....Maintenance is very important even if it is just a lawn mower.....