Purina Beneful is horrible food. That could be the problem right there.
Pet food from the grocery store is made from the waste from the human food industries, the not-fit-for-human-consumption by products. Please read this article to find out what is really in your pet food:
Get The Facts
Read this sticky to find a better quality food for Stella. I bet you will see a huge improvement in her coat:
http://www.yorkietalk.com/forums/yor...e-samples.html