Cause

No or insufficient grease on the teeth of spline hub or grease is vanished under elevated temperature due to the use of low heat-resistance grease.
Because of the rust, the disc does not move on the main drive shaft smoothly.
Then the clutch disc is not disengaged fully from the friction surface.

Solution

Apply grease with high heat-resistance to the spline of both main drive shaft and clutch disc assy.
Insert the disc to the shaft temporarily, and wipe out the excess grease.