Many dogs will wolf down more than is good for them and look for more. Others refuse to eat more than than they need. Evaluate the dog. You may want the vet to confirm your judgment. Adjust the dogs food and exercise as needed to reach its ideal body condition. Some German Shepherds and other breeds may refuse to eat enough to completely hid their ribs. As long as you are feeding a concentrated, meat based chow, the best thing is to accept it.
The worst thing you can do is to bribe a dog with rich foods into eating more than it needs. Instead, Put down the dish with what the dog should eat, and give it 15 minutes to eat. Then take it up. Do not give it anything to eat until its next scheduled meal. In a few days, it should be eating what it needs. Continue to check its ribs and adjust the food as needed. This is not easy. I had a Shepherd go 3 days on a few nibbles. I was a wreck, but she was fine. It is almost unknown for a healthy dog not to eat what it needs. Unfortunately, in too many cases, it is less than the package says, and less than the owner thinks the dog should have. Many dogs are quite good at holding out for tastier chow. Like kids, sometimes it calls for tough love.