When I read about your problem, it sounded all too familiar. My Newf puppy (at 12 weeks old) who was also on Pro Plan large breed puppy food, suddenly stopped eating. Nothing I did would interest him in his food. The breeder suggestions I received were the same...put the food down for 10 minutes, then pick it up. (the idea was, he would see that I took it away, and be very interested in it next time) Well, this did not work, nor did adding chicken, broth, cheese, or yogurt. I tried everything. I was so upset because he just kept loosing weight and not eating. Finally I took him to his vet where he was diagnosed with anorexia.....y es, anorexia! The vet prescribed medication that stimulated his appetite. This worked very well (except the pills made him VERY hyper) He was on meds for about 2 weeks, then his appetite picked back up, and he has been fine ever since.