My Scotts 17hp 42" rider just started to act up. When I depress the clutch and shift gears, eg from forward to reverse the transmission will not catch when shifted into the new gear. But if I sit on the seat and rock back and forth so that the wheels move slightly, the transmission gears will catch and the rider will operate properly until I depress the clutch once again and shift into another gear.

I put the machine up on ramps and can see nothing wrong with it. Is there a way to change transmission oil or some other form of maintenance that I am not aware of? The unit was made by John Deer and was purchased at Home Depot about 10 years ago. I use it only to mow a level lawn. Any ideas as to what the problem might be?