If your dog is allergic to chicken Fortiflora is not the best choice - the the ingredient labelled "animal digest" comes from chickens. I phoned the company to check on ingredients since my little guy gets bloody diarrhea from chicken and that is what they told me. It can be hard to find a digestive supplement that doesn't contain chicken. After a lot of searching I found one called "NaturVet Digestive Enzymes"
Digestive Enzymes with Prebiotics & Probiotics
This has really helped with both our chis tummy issues - no bloody diarrhea for the little guy since he started taking it a year ago and much less grumbly tummies from both of them. As a bonus it much cheaper than Fortiflora and I can buy it in our local pet store.
Also, if gas is the problem you can give them "Oval" (generic is simethicone), an anti gas medicine for babies available in the drugstore. Our vet told us that it is safe and almost impossible to overdose with it. We give about half the baby dropper.