I just looked up Pro Plan on dogfoodanalysis.com and according to them, it is a one-star food. So, it isn't very good. I would recommend looking for a five or six star food. You can go to the website I mentioned above and see which foods are the best. With such small little bodies, yorkies need optimum nutrition.
Like I said in an earlier post, we recently switched to homecooking and all our eating problems disappeared. I fed my three-year old Science Plan for the first year and only later discovered that it wasn't a good food. I feel really bad now for giving him that kibble exclusively for so long. No wonder he didn't want to eat.
Anyway, maybe look into another food, but otherwise it sounds like you have things under control.
