Has your vet diagnosed the problem - food intolerance, allergy, colitis? Things to bear in mind:
1. dogs aren't supposed to eat complex carbohydrates (potatoes, rice, wheat, oats and other cereals) as they have short digestive tracts and don't produce the right chemicals in sufficient quantities - all commercial foods contain complex carbs, albeit that they have been heavily processed to make them more digestible. Why? Because they are cheap "fillers". The reason Chappie works so often for dogs with digestive problems is that, if you read the label, you'll see a horrifyingly low meat content and the whole food has been processed to within an inch of its life;
2. if your vet hasn't done lab tests on your dog's poo, you should get a fresh sample to him/her early in the day, so he/she can send it off for analysis. Even if it's colitis, it could be triggered by bacterial or parasitic infection;
3. if infection is ruled out, and it is colitis (as well as diarrhoea, you get a shiny mucus coating and occasional "spotting" with fresh blood in the poo), then it does come down to diet management and occasional interventions with tablets.
If Burns doesn't work, other foods to try:
(i) NatureDiet - a wet food, with (for commercial food), good quality ingredients. They also do a product called "naturezyme" - an enzyme-based powder you add to the food, to aid digestion;
(ii) Forthglade - they've done a food with a very similar profile to NatureDiet;
(iii) BARF - I switched my dog, at six months old, to BARF after four months of colitis-induced problems and I wouldn't go back to a commercial food now if you paid me (even if Paddy was "cured" of colitis once and for all).
Good luck with the Burns.
Denise