I once had a car with 4 speed, to which I added a high performance clutch. So it had more pressure and a different compound (more steel) on the disk. OVERNITE, routinely, it would rust together if it rained, gauranteed. I could push the gearshift into gear, THEN start the car and it would instantly be moving, off I'd go. It would free up eventually while driving. This clutch did not slip, ever. Even at 50 mph shifting into 3rd gear, it would lay two patches of rubber for 50 feet. Fun for around town driving.
I think your clutch is rusted stuck together. Taking the tractor apart will not help. It is going to happen again if I am correct.