Could very well be food allergies. What food are you feeding and does it have corn in it? Many people say dogs are allergic to corn, but it may well be the way the corn is processed and put into the food that is what they are allergic too.
From
Dog Food Reviews | Dog Food Ratings
more specifically from the review of Beneful (what my foster was on before I got her)
Beneful Dog Food (Dry) Rating and Review
"Corn (as it’s used in most pet foods) is rarely of human-grade quality. Although there’s no way to know for sure, it’s reasonably fair to assume the corn used here is probably the same kind used to make feed for livestock. Second-rate grains like these are almost always contaminated with insects, their droppings, their expired carcasses, and grain mites along with their ever-present toxic molds."
Many associate corn with chronic canine allergies. But the truth is, those allergies are probably more the result of reactions to what’s “in” the corn… rather than the corn itself.
Corn is definitely not a quality ingredient."
Some "higher" quality foods still have corn in it, such as Royal Canin.