Loves Labs ~ like jesse's mommy asks, what are you feeding? Some foods are deficient in a particular nutrient your dog needs. I would also ask whether you have changed the proportion of food or introduced a new kind of treat. Some foods can actually work against each other by preventing the absorption of vitamins and nutrients. And sometimes when dogs are "put on a diet" they look for ...well, let's just call it "filler"
Finally, is anything new in their environment? Sometimes a change of homes or changes in existing family structures/ dynamics can cause stress which manifest in blechy