What can I do about dog losing hair on tail?

My 7 year old Chihuahua keeps biting the hair from her tail and backside. I keep taking her to the vet and I've gotten answers from allergies to impacted glands to yeast infections to heredity but no consistent relief for her yet. The last time he didn't even send antibiotics home for her (she bites and makes sores on her skin), but gave her the routine shot in the office and some prescription shampoo and leave on conditioner. Well now her tail has bloody sores all over it again. She's been spayed and is on flea treatment so I can rule those two issues out. Her mother and sister did had similar problems so I do wonder if it's hereditary. Anyone else have any experience with this? I know it hurts/aggravates her and she looks pitiful. Even if it is just bad genes shouldn't there be something to help her?