What are they giving her? The bacteria may be resistant to the ones she's taking, or if she's had it a long time, she may not be taking them long enough to kill all of it. Doctors are famous for only giving a 5-day supply of antibiotics, maybe 7 at the most, but sometimes you need to take them for two weeks, if not longer, especially if she keeps testing positive. I have found that the only one that works for my infections is ampicillin. If they've been giving her cipro or bactrim, some people on here say they don't work very well. I think I would suggest to the doctor to not only try something different but for a longer period of time. Could be that the infection is just that bad and needs longer to be cured.
Good luck, and let us know what happens.