Thanks, Mordy, for reminding us that most external problems have an internal source! Don't you wish you could make the companies that manufacture all that awful pet food shoulder the vet bills they create?
It can have something to do with diet. Our dogs are on natural kibble/raw and the glands on the one are doing a lot better. Only every couple of months does something "happen".
My dogs have never had a problem either. Maybe because they are still young. Or like mordy says - because they do get a lot of exercise and relatively good food