Have you ever tried Psyllium Seed Husks? It comes from the Plantago ovate plant which is rich is soluble fiber. Funny thing about psyllium is that it is used for constipation OR diarrhea! It can stabilize bowl movements in some pets/people. It also helps to detox the colon! You can start out with 1/4th teaspoon and then work up to the amount that keeps your dogs stool steady. Once using it, if the increase of the psyllium begins to loosen the stool up again, back off to previous dosage, hold there and watch.
Here is an organic, gmo free product: Buy Non-GMO Organic Psyllium seed husk (Plantago ovata) - Oregon's Wild Harvest