Remove the cassette lock ring and ensure that the cassette overhangs the freehub body by a couple of millimeters, so that the lock ring clamps the cassette without bottoming out on the freehub. If it does not overhang then additional shimming is needed. Then grease the lock ring threads and torque to the recommended value , typically 22-36lbf-ft or 30-49N-m, which is a surprisingly-large (to me) torque.