What do you feed your dogs? I've got mine on Taste of the Wild, rice, and wild game meat (cooked). Keeping my dogs happy and in prime condition has always been tricky for me. A lot of Nihon Ken have sensitive stomachs and allergy issues, and some are just picky eaters.
Baron is a picky eater, with a sensitive stomach. The summer heat does a number on him every year, and he loses his appetite. During the hunting season, it's nearly impossible to keep weight on him as he burns through more calories than he eats. My Shikoku females have great appetites, and are a lot less picky about what they eat.
I've learned that staying away from foods with corn does keep my dogs' digestive systems happier. I'm not fond of kibble in general, as I'd rather feed my dogs a more natural and less processed diet. Unfortunately there are many times when my schedule does not allow me to cook for the dogs, and switching foods randomly upsets their stomachs a lot. Way back when I had 1-2 dogs, I did pre-cook/freeze food for them, and naturally they loved it.
There are many discussions on the internet regarding optimal canine diets and dog food, so if you haven't given too much thought to your dogs diet other than to try to pick up the most expensive food you can, google it! There are a couple sites that give reviews on a lot of the kibble out there, and here are two that I use
If you feed your dogs a lot of meat, stick to organs and red meat, and stay away from too much fat. Believe me, it wrecks their stomachs, and you will not enjoy the results.