Have you noticed her licking the furr off? It so,it's probably something bothering her and it's her way of dealing with it. In this case, you have to try and narrow down some stress factors and get to the bottom of it (no pun intended).
It could also be an allergy weather it's seasonal, inhaled or a food allergy. Some vets will just give steriod shots to help temporarily. I personally don't like the idea of injecting my cats with steroids ever few months. I would talk to the vet about allergy testing. They can narrow down what it causing the allergy and you can try and take it out of her enviroment.