One of mine has a very sensitive tummy and has loose stools (or worse)
when she eats a food that doesn't agree with her. She tends to have the problem mostly with the holistic brands.
I found a very good (IMO) food that is formulated for sensitive tummies that works well for her. The problem you are describing doesn't seem to be all that uncommon for Yorkies, unfortunately. Just hang in there and keep trying--you'll find what is right for her.