Boo is one of the pickiest eaters I have ever seen. I'd just get so frustrated with him. Like you, I knew he was hungry but some days he'd only eat a bite or two.
Two things that changed that seemed to fix that. 1) Gracie Ruth came into the home and we changed our routine to free feeding instead of scheduled times. When she would start eating it seemed to fuel him into wanting to eat also. 2) The kibble she was eating seems to be one Boo actually likes. We must have tried a dozen different brands, served a dozen different ways. The brand Gracie was on Boo will eat straight out of the bag, no doctoring needed. Yeah!
So I'm thinking you need to get a 2nd Yorkie to entice your first one to eat.
And as much a pain in the rear as it is to keep changing foods, keep trying until you find one she likes.