Why does my cat fart? We have her on Purina One Sensitive Stomach dry food and half a can of fancy feast wet.?

Her ping gas can clear just about any room, the smell doesn't go away for awhile. She is very active and playful. Our other cat has to be on wet food so we can't stop her from eating it. The vet says that it was from the previous dry food we had them on but it has been a few months and she is still farting. It's gotten better but not gone away. Also her stool is not solid. Any other suggestions?