I feed Primal too. My Wylie, who is about 7lbs, gets 2 nuggets in the morning, and 2 in the evening. His body condition (weight) is exactly where I want it.
What about trying green bean as the treat? Or some other kind of veggie? And or, something to chew on.
Here is primals feeding calculator, where you can enter in different options for your kiddo:
Feeding, Transition and Safety for Raw Dog Food Diets