I have an 8-mth old shi tzu and within the past few months he has become really unexcited when its time to eat his food. However, he seems to like eating everything else. I have changed his food, the original food he wouldn't even touch. Now it seems as though he's tired of this food also. I've been feeding him Purina One for puppies. Does anyone who has this breed have any suggestions on what I should try next?