Cause

Slipping:

Not enough friction area causes less torque transmittance and results in slipping under heavy load.

Premature wear:

Reuse of worn flywheel is same condition as worn clutch disc, and it results in premature wear.
If you resurface friction surface only, it causes premature wear as well.

Solution

1) Replace with new clutch.
2) Use flywheel which has the same dimension as new, the height between clutch cover mounting surface and friction surface shown in figure.