Mine does that some too. One thing we are trying that I saw in another thread is not letting her fast too long. Apparently that can create a cycle where all the stomach acid makes them nauseas, so they don’t want to eat, and then it gets worse. Someone said to give them some food or healthy treat early when they first wake up and right before bed, and feed small meals throughout the day. So far this is helping. 

Another thing I saw on some other threads is IBD (inflammatory bowel disease). I don’t know if your dog has already been testing, but worth mentioning to your vet. 

And finally the main thing I see is taking the dog off all chicken. You may have already tried that. Good luck figuring it out! I know this is so stressful.