How old is she now? One of mine had something like this at about a year old, in warm weather and he looked like a mangy camel. I gave him a pinch of linseeds every day and not only did it all grow back but it was lovely and thick and glossy. I was advised it could have been a moult. Although it's not summer at the moment, with heating on it might be warmth causing a moult maybe.
I would definitely give a pinch of linseeds every day though - it works wonders for fur condition and growth

As you probably know there are so many reasons for fur loss. If it's not mites or cushings, it could be a moult. Another cause is lack of protein in the first year, but it probably isn't that as you give her a good diet.