Sounds like Jetta could be suffering allergies of some sort or yeast. Are all of her basic blood tests normal? Sometimes skin and coat problems can be secondary to a health problem.
Allergies are usually environmental, although diet changes can help.
Have you taken her to a dermatologist? The derm specialist is often able to figure out problems that a regular vet cannot.
Here is a thread that may be of some help -- Wylie's Mom is a great resource for derm issues (ignore that Fleas are in the title):
http://www.yorkietalk.com/forums/yor...ease-help.html