I have a john deere jx 75 walk behind self propelled mower - the mower's transmission/self propell does not work. When I engage the self propell handle the mower will not move but when I raise the back wheels up off the ground they spin and when I lower the rear wheels back to the ground and engage the self propel handle the mower will not move. the transmission has been replaced twice and the mower is only used once a week during the summer . any ideas on what to do .