This is very common on improperly kept uros and what usually happens is it turns into a bacterial infection under the old layers. A lot of times treating with an oral antibiotic is not successful due to the poor blood flow to the specialized tail scales. I would get her to a vet and have him/her try to culture it and see if it is bacterial and if so what will kill it. Then treat it topically. I have heard that silverdine(spelling) works ok on this but it is a very long process(up to a year). I think that if there is an infection you may have to continue debriding the rest of the tail as you have done a long with treatment of the topical. Keep the cage absolutely dry and sterile. Like Torey said a little humidity is ok but no direct dampness. Usually this problem happens from malnutrition and keeping the relative humidity too low in the hides not allowing the skin to shed properly. I would definately get her to a good reptile vet.

Good luck
Tom