
Some dogs have anal gland problems no matter what you feed them. I have one that does and she's been on various diets. Best thing I found for her was Naturediet with some added wheat bran (a level tablespoon). I'm off Naturediet now because of all the supply and quality problems.
I tried JWB duck as well, but shortly after I started it she had a bad attack of diarhea that necessitated two trips to the vet. So I dropped the JWB, even though I'm not convinced it was the cause. It may well have been something she scavenged or the last pack of Naturediet was bad.
I switched her to Arden Grange, the regular variety. In any event, I've noticed she's back to having more "issues" with her rear end

even though I've been adding extra bran or a whole bran cereal.
My other dog is fed exactly the same thing as my bitch - and his poo is always hard and never has had anal gland problems. Like I say, it's not necessarily the food, some dog's simply have different metabolisms. True some foods will work better than others - but don't expect a complete fix to the problem by diet.
I always add extra oil to my dog's food during the Winter, it stops any dry skin problems caused by heat in the house. I just add a capsule of Evening Primrose oil, which does the trick. It's extra calories as well ... not that mine need them!