Bella's delicate digestive system

Can anyone advise what we should do. We believe our staffy is about 4 years old. We have had her for three years and she came to us as a rescue with nothing known about her background. She is fully inoculated and is generally in good health. She has large build up of wax in her ears but we clean them out and so this is not a problem. She was spayed as soon as we got her. The problem is that her tummy is just too sensitive. If she “steals” a potatoe, piece of bread or vegetable etc, even a really small peice, it gives her the runs. I have just this week moved her away from tinned dog food to dry food to see if this helps. I have had dogs all my life but Bella is my first staffy. i have never had one with such a sensitive tummy. Is this normal?

Thank you

Sam