I agree, thyroid testing would be my first action.
Our kids have all been on Eukanuba Lamb and Rice for years, but we've become disatisfied with Eukanuba since they were acquired by Proctor and Gamble. They've decreased the size of the bags, increased the price, changed the formula (they have so many "breed specific" formulas now you can hardly even find Lamb and Rice), and quit selling at dog shows. So we've switched to Nutro Natural Choice. For our easy gainers, we add 1/3 to 1/2 cup of green beans to their decreased amount of dog food, adjusting according to the visibility of ribs! All but one of ours like the green beans, which we buy in the giant cans at Costco or Sam's Club.
Different metabolisms also figure in, just like people, I think. One of our girls stays skinny and gets almost 2 cups a day! She just burns it right off. Another of our girls, with the same amount of exercise is lucky if she gets 3/4 cup per day and we're constantly fighting her pudginess…
Terry