Why does my puppy have an bald spot on her back?

my 7 month old cairn terrier/bichon mix developed a baldin (not completly bald) on her back about a month ago. It looks a brownish color and is flaky. She was tested for mange and mites which was NEGATIVE, and given an antihistamine and a fatty acid supplement with the dx of dry itchy skin. There has been no improvement at all.... still itchy, flaky, sometimes irritated looking if she bites at it..... im not sure what to do now. the vet has to get back to me .....i was thinking allergies or a flea bite ??? any thoughts or suggestions what this could be or what to do?????