lol, Yorkies are known to be picky! Jackson was the same way! It took him a while to find a food he liked. But honestly, and I mean this in the nicest way possible (I'm usually not one of those you should feed your dog this-kind of person) but Purina isn't a very good food at ALL. And maybe it's upsetting his tummy in some way even if he doesn't show you. Maybe not. But personally, I wouldn't feed it. The first four ingredients are Whole grain corn, corn gluten meal, whole grain wheat and chicken by-product meal. All awful for dogs who are naturally carnivores. However, I'm not sure which food you're feeding and there is no wet foods listed on the site below, so I dunno if yours has different ingredients.
You can see this website
Dog Food Reviews - All Products - Powered by ReviewPost
with all the different Purina foods, rated one star. That's just one websites opinion but I think Purina has been recalled quite a bit.
Jackson tried a bunch of different foods, lower rated ones as well as higher rated ones and he finally loves Merrick's Before Grain line of food - they have dry food and wet food and I mix in a bit of wet in the morning. They have three flavors - Buffalo, Chicken and Salmon so I switch it up for him every once in a while so he doesn't get too bored.