Have the vets done a swab ?
Bea had an infection that wouldn't clear up so the Head Vet looked at it under a microscope, he said that she had "a lot going on in there" she had around 3 different germs etc, he then gave her a different drop which cleared it up.
They will always go for a drop that will clear up a General infection and sometimes this just doesn't work, it also ends up costing you more