I really don't think it has anything to do with shampoo. Shampoos and conditioners have some affect on the hair, but can't turn silky hair into wiry or visa versa. A low thyroid can make hair grow coarser and the dog will also have thinning hair, and even a weight problem. The problem, however, is usually genetics, and most breeders don't breed to standard, but breed whatever dogs they have. The Yorkie coat with the proper coloring and silky texture is a very difficult coat to achieve, but I always encourage people to support those breeders who believe that breeding to standard is important. This link gives information on coat types.
http://www.yorkietalk.com/forums/gen...rt-thread.html In your case, I would suggest a vet check, there could be an underlying skin condition that is affecting hair growth.