There are several causes of hair loss in ferrets. First, ferrets usually shed their coats twice a year, in the fall and in the spring. Ferrrets are extremely sensitive to light exposure, so this pattern can be changed with artificial light. I'm assuming that what you are talking about is more than just shedding, and that your ferret is not only loosing her hair, but not having normal regrowth.

In female ferrets that have not been spayed, there is a hormonal condition called hyperestrogenism that is caused by the female being in heat too long. The high levels of estrogen can cause hair loss. This can be serious since a condition called aplastic anemia can also develop, which can be fatal. If your ferret has this, she should be spayed.

Another cause of hair loss in ferrets is adrenal disease. Adrenal tumors are fairly common in ferrets over 3, and may grow on one or both adrenal glands. Usually a patterned hair loss is seen with adrenal disease, with hair loss starting at the base of the tail and spreading up the back and then down the tail in a symmetrical pattern. The ferret can eventually lose up to 90% of its hair. Most tumors are benign, and the common treatment for this is surgery, which has a good success rate.

Another major cause of hair loss in ferrets is allergies. The two most common allergies are food allergies and topical allergies. Food allergies usually cause digestive problems in ferrets. Topical allergies are irritants to the skin. Ferrets can be allergic to detergents, cedar shavings, corn cob bedding, household cleaners, or have bug bite dermatitis. It's important to identify anything that might be causing a topical allergy, and treat any parasites that might be causing problems.

I would recommend taking your ferret to see a vet. Hair loss can be a symptom of a serious problem, and it's best to get it treated as soon as possible.

Let me know if you have any questions or I can clarify anything.

Here is a link to hair loss in ferrets:
http://www.ferret-fact.org/Shedding.htm