Possibly she has a flea bite allergy. The stronghold will kill the fleas but only when they bite. So flea saliva can still cause a reaction. I have recently taken one of my ragdolls to the vets as he lost all his fur at the base of his tail due to a flea bite allergy. He has no redness/scaliness either, but after a course of steriods, his fur has started to grow back. I giving him viacutan as well to help with skin conditioning.
Has she been through some kind of stress recently ? Like a major change in routine perhaps (Psychogenic (neurogenic) dermatitis.
Other causes include ringworm, cushings disease, dietary intolerance, mange, Erythema multiforme, Hyperthyroidism, and atopy.