Have you spoken with her breeder? Is there a chance she was blue born?
"Dogs that are born blue (silver or gray) carry the dilute gene of dd, which should NOT be in the Yorkshire Terrier breed because it causes the "Blue born puppy." It is a genetic mutation in color. It has been proven that the blue born puppies do have skin problems and the hair does not grow."
Parti yorkies - PARTI YORKSHIRE TERRIER CLUB