I've heard tell of acouple of the earlier SERPA holsters which had issues with the retention lever... It wasn't especially common though.Itis likelythe spring in the mechanism which has yielded. Best bet would be to contact Blackhawk:
BlackHawk Products Group
4850 Brookside Ct
Norfolk, VA 23502 U.S.A.
Maybe they can send you another spring with installation instructions, or replace the holster.