You say you had the clutch changed, are you still using the original dual mass flywheel.
Was that checked over before fitment? If the surface of the flywhel that makes contact with clutch friction plate was scored or not even that could result in some slippage?
Was the secondhand clutch in like new condition, if it wasn't run in correctly, the friction plate may have some glazing that would prevent it "gripping".
Have a word with the person who fitted the clutch to confirm above thoughts?