I thought it might be an issue with her teeth as she doesn't seem to like hard food - we buy rat supreme to stop the selective feeding thing that they do. But her teeth look just the same as her sister. She seemed to be losing weight and being very lethargic and not eating. But when we give her banana and melon she eats them very happily (and cheese...tut tut!).
So we are making sure she comes out and gets given the food she likes at least once a day.