One of my foster's is suffering from a very similar skin condition also. Two different vets have diagnosed him with irritations/staph infections due to the fleas he had when we got him from the shelter.
He has been on antibiotics but each round has only been for 10 days. After reading the article from the link below, I'm thinking he wasn't on them long enough to get rid of the staph infection. Possibly the information could help in your Belle's case.
Bacterial Infections in Dogs | Dog Bacterial Infections - by Dr. Carol
It's also possible she has an allergy to the shampoo you're using, which is why I suggested the oatmeal shampoo. It will alleviate any discomfort and rarely causes irritation.