Diamond Naturals Extreme Athlete is what my hunting dogs have always gotten. They eat it like crazy but they'd eat anything if it was put in front of them 

Yours aren't extreme athletes but yeah something is up if they're not eating. Vet checked them lately? 

I know of one spoiled dog that was a slow eater and the owner put some plain yogurt on top of it. That worked and the dog ate all the food. 

5 years later the owner is still putting yogurt on the food for every meal, so be careful what you do