First, has your vet tested for campylobacter? This could cause the symptoms.

If there is no campylobacter, it is likely to be a food intolerance. The commonest ones are to wheat, soya, less often maize and poultry - but it could be anything. You will have to experiment. You could start with lamb which is usually safe, or white fish, cooked with brown rice, add a vitamin supplement. Your pet shop should be able to get you a box of frozen minced lamb for dogs - or better use fresh human quality if you can afford it. Feed that for a few days and see if there is any improvement. At first you will probably have to give it extra time for any irritation in the gut to settle down. Once you have found a basic diet that doesn't cause bad poos, start gradually introducing new foods, different meats or vegetables, just one at a time, and give it a couple of weeks before trying something else new. If anything triggers the bad poos again, go back to the basic diet and let it settle again before trying anything else.

That there was an improvement with JWB is a good clue - it is gluten free. I feed my dog another gluten free food, made more palatable by mixing with a gravy made from Webbox chubs (those plastic wrapped sausage-shaped tubes of food, from Asda, much cheaper than cans) as it's gluten free too, mashed up with warm water, then mix in the dry food so it's all coated.

Drinking dirty water could also be a trigger. Don't let your dog drink from puddles.