I think in general dogs can be sensitive to diet changes particularly if you change the brand. Usually (in the US) when changing to a new diet, it is recommended that it is done gradually over a period of a week or so. One of the few fool proof ways to "see" how your dog is doing is to check the poop. By changing the diet you are making this harder to read.

Like the other two posters I feed my dog the same base food as at home. But I do add to it foods that we catch on the way. The amount is 1/3 more than at home because she is working longer hours back to back days.