Do you of did you have an oil leak at the rear main seal or the oil pump that could have gotton on the disk?
Has the tractor been sitting for a long time?
Once it breaks loose, does it then operate and feel proper?
Oil on the disk, sitting a long time will act like glue. It will have to be burned off with some abuse.
A rough spline or bent spline or a spline that is badly worn will want to try to hold the disk from moving.
Have you ever had it apart?
Just some things to think about.
