Our GSD just turned 6 months, and he seems to always seems to have a lack of appetite after eating the same food for over a month. We've fed him Acana (which he couldn't handle because of the protein content), then we switched to Now, which he was good on for a while, then stopped eating. Then we switched to Performatrin and he had a healthy appetite for a month or so. And now he's been refusing to eat for the past few days. We fed him some leftover Iams that we got for free to see if he will eat it. His appetite suddenly came back and he finished 4 bowls in a sitting!
We are taking him to the vet this week to see if there is anything physically wrong with him. But it seems that he just gets bored of the food.
Our vet always tells us to add yogurt to their food if they have a lack of appetite. However, I don't want to have to add stuff to it to entice him to eat. Our trainer also says it's a bad idea to have to add special sauces/yogurt to make them eat. We usually take him out for a walk before meals and we always make him wait before feeding him to make him "work for it".