There's a few things you can try...
1) Try a food with a higher concentration of Omega 3's and 6's, Like
Orijen Six Fish or
Acana Pacificia.
2) Try adding
Salmon oil to the Diet, most Salmon oil normally have a lot of Omega 3's that what most pet food lacks.
3) Add
digestive enzymes to help your yorkie absorb all the nutrition in the food.
4) Switch to a Soap Free shampoo, something like
TheraNeem