We had the same problem. The first time dd got a yeast rash she was about 9 months old. It was awful. We used the Nystatin and a steroid cream. It did finally go away after several weeks. The next time it came back, I knew what it was so I was able to get control of it. Over time I tried to get away from the Nystatin. I tried everything, over the counter yeast treatments, natural remidies, different brand name diaper creams.
One cream that works like magic, so long as I catch the break out early, is Gaia's Children diaper cream. It has a wax base, altough it feels like clay. It works wonderfully and I have tried many creams, natural and synthetic, to get rid of the yeast.
Also, try putting her on some probiotics, they help to get rid of the yeast from inside out.
Finally we decided to cloth diaper when my dd was 14 months old. It is never too late to start.
That has made all the difference.
We have not had a rash since.
I think that it is the best solution to persistant diaper rash.