I think you will find that the problem is rust on the input shaft and pressure plate, which keeps things from moving freely. The best solution is to use the tractor and work the rust off using the clutch. If it won't free up, the only solution that I have found unfortunately is to split the tractor and wire brush, etc. the rust away.
If you are going to park it for a while, make sure to block the clutch pedal down some via a 2x4 pressing on the seat or something like that.
Good luck!