Were you able to flare the lead out over the flange or is is just sitting there? A picture would help. Often, when replacing a mud bed with ply and cement board, the floor ends up being thinner. If there was enough lead, the better thing would have been to use another brass ring and flare the lead out over it. Not sure without seeing some pictures, and then, probably best for one of the pros to resolve it for you as to what your options are.