I think they may be a problem because buying a big bag means it lasts a long time with one little dog, and even attempting to keep it stored fairly well, it is bound to get a bit stale. Maybe the dogs know when it is getting rancid or whatever.
I hadn't thought of it before now, but maybe they should have 'eat by'dates once opened.
I am coming to the end of an Eden bag, and one of mine is less impressed. Putting a bit of cat food with it works !