It may be the breed/s in them. Northern breeds like huskies will often save their food, going for long periods of time without eating, and still maintain a great deal of energy.

It sounds as if they are free-fed. Some dogs who are free-fed develop the habit of not eating because they know that food will always be there. If they are fed meals, and have the food taken away after 15 minutes, they may start realising that they only have one chance to eat so they'd better do it. Alot of dogs are also in tune to what they are allergic to and will refuse to eat a food they know they are allergic to.

I had a huge problem putting weight on Visa when I fed kibble. I tried plenty of foods, and she would eventually stop eating most of them. The ones she didn't stop eating I wasn't satisfied with because they didn't help with other things, like dandruff or bad breath. So I switched to a raw diet, and things have been spectacular since.

Try looking up the recipe for Satin Balls.