Small Engines (Lawn Mowers, etc.): John Deere RX95 shifting problems, input shaft, pulley

Question
My JD starts fine, but it doesn't shift. Reverse/forward won't engage while mower is running, but I can engage them when it's off (or at least when I place in neutral, I can move the mower forward and backward, and when in a gear, either forward or reverse, the mower is locked in). is this a clutch issue..would this be a straight-forward fix? I am mechanically-inclined, it's just that this is my first rider...

Answer
Is your writer equipped with no mow in reverse?  So when you shut it off does it operate properly?  

If not then it could be the linkage or belt placement.  Has the unit operated properly?  If it has not remove the deck take off the belt's and start over. there should be a belt going to the transmission input shaft it should come directly from the engine to the transmission. the belt will be tensioned by a small pulley that would move when the clutch is engaged. if it's a flat idler it goes on the outside of the belt if not it goes on the inside of the belt.  when you have that attached try it   ride it around in all the  gears then reinstall the deck per your operators manual. I hope this helps