Fur mite/fungal infection (rabbits)?

Hello, I have a question on my rabbit.. Around two months ago, I noticed some kind of skin irritation on her back, like it was big... My mom finally took her into the vet, they weren't much help, they said there was nothing wrong. So then we went to another vet. . . They did a skin sc and didn't find any mites under the microscope (He did this a couple times) Then, they checked her for a fungal infection...Both negative...So my question is well... What's wrong with her? I mean what else could it be? Could it be extremely dry skin or something? She's close to a heating duck in the house, but could that really cause her that much damage? Please help.