Check for a burned or a glazed facing caused by oil on the facing.

Correct the leak at its source and install anew clutch disc.

Check for worn splines on the input shaft.

Replace the worn input shaft.

Check for a warped pressure plate or a warped flywheel.

Replace the warped pressure plate or the warped flywheel.

Check for burned or smeared resin on the flywheel or the pressure plate.

Sand off the burned or smeared resin if it is superficial. Replace any burned or heat-checked parts.