Hi and welcome to the board
We had a similar problem with one of ours. His issue is not a common one, but may be something to look into if you've exhausted all possibilities. It's called EPI (Endocrine Pancreatic Insufficiency). There are 3 digestive enzymes created by the pancreas. If any one of them (or all of them) are low, even the low normal range, it can cause issues. Our boy is deficient in Trypsin, then enzyme that digests protein. He has to be on a supplement called Pancreas-V. It's rather expensive though, $70 for a 100 gram bottle. It's a powdered Porcupine pancreas. It digests the food before he eats. There are several different symptoms. His were loose stools that looked like "cow pies". They were very light in color and he was literally starving to death. He would also throw up quite a bit as well. This is a link to a good website:
After reading through it, if anything looks familiar, talk to your vet. A lot of the sites I have read said that "most" vets never think to test for it and it goes unnoticed in a large percentage of dogs. Luckily, our vet is a very bright lady and has years of experience with complicated dogs, so she thought of it right away. It's a simple blood test called a TLI (trypsin-like immunoreactivity) and requires a 12 hour fast, but that's about it. One other thing to consider is Colitis. Our old boy has had Colitis his whole life and has had trouble with diarrhea and constipation (rotating). We only got a diagnosis 2 years ago, and he's almost 12 now.
Hopefully you can figure out what's wrong and remedy it! Please let us know how things go, and pics are always welcome!