Have teh mechanic check t be sure that the cassette is far enough out from the hub face for the lock ring to engage it properly. The cassette needs to overhang the end of the hub by a millimeter or two so that the lock ring can clamp it securely. It is possible that if you have a wrong or missing spacer (such as is required for a 7-speed cassette on an 8-10 speed hub) tightening the lock ring will not have any effect.