[matese]

Originally Posted by Bluebells

There are many, many possible causes: allergies (food or environmental), parasites, skin problems, endocrine diseases, infections, etc, etc…. It might help if you post more details on the types of tests that your vet has done already so anyone reading your question knows what has been ruled out. Also, possibly photos of the skin at the sites of the hair loss, in case it looks familiar to anyone.
Seeing a dermatologist is probably the best suggestion.