Yes we need more details. How long has it been this way? Did it just happen or has it been for a long while? How long on what kind of food? You can give 1 tbsp of canned 100% pure pumpkin (not pie filling) a day to help firm it up but I would more wanna get to the cause of the problem.
Iams is not a very good food. Check out the dog food ratings, Iams is filled with corn and grains- which are not good for bulldogs. (Or any dogs) Click the link below:
Inconsistent stool could mean several different things. If she has diarrhea you should keep a close eye on her. But if she does not- it could be allergies or table food, treats, ect. I would trying giving her some good dog food and see the difference- not only in her stools (less waste), but in her overall coat and appearance.