Honestly I don't know what makes a GSD stop eating. I've had lots of breeds from herding dogs to sighthounds to terriers, and I've never seen anything like it. I've gone through it with every
Carly, at 4 years old, will finally eat anything that is not nailed down.
Russell, at 12 months old, has his days where he looks at me and his food bowl like I'm crazy. I tried to feed the dog pre-ground raw (Bravo) last night and he absolutely refused to eat it. He wanted his Fromms with the canned tripe (so, yes, I gave it to him).
Sage, at 3 years old, is still hit and miss. She'll be going along really good, and then will refuse to eat. At all. And if I try to stick any sort of supplement in it, forget it. I'm currently showing her, so I can't afford for her to lose any weight. The one thing she will always eat is that refrigerated Vital by Freshpet. Comes in a roll. She loves the stuff. So, I chop it up and mix it with her kibble.
So to summarize, I have no idea what the magic trick is. I don't fall into the camp of making them eat dry kibble no matter what. I don't mind adding stuff. As for supplements, I mix them with the Vital, or in a meatball of ground turkey or beef. That will usually
work. Mostly. Darn dogs.