Have you tried adding digestive enzymes to her food? My cat is about a year old and had a really sensitive stomach since the day we got him. We tried a bunch of different foods and the vet gave us FortiFlora but nothing really helped much. We started adding digestive enzymes to his food and it helped him SO much. There are a bunch of different brands, but this is the one we use:

http://www.amazon.com/gp/product/B0002ADJYM