My Molly can't digest the carbs in dry food, and I wonder if that's the same problem your kitty has. We found Molly lost on the streets when she was roughly 5 months old, and for the next year, she had constant diarrhea (I think the only firm poop we saw come out of her was the first night we had her). She was eating premium kibble food, like California Natural, Felidae, Timberwolf Organics, Innova, etc., but nothing worked. It was always diarrhea, and bad diarrhea too, where it would splatter everywhere on the walls, and the smell was enough to make you die.
But a year after we found her, I decided to heck with it, why not just take the plunge into wet food? And as long as I'm going to try wet food, why not try RAW food instead of canned? So that's what I did and miracle upon miracles, her poop problems cleared up! Now she has firm poops with no smell at all! The only time she gets runny poop now is if I give her something foreign (like a hairball remedy, or some dry treats, or anything with carbs, basically. She's also sensitive to beef, which I've suspected ever since we found her). Dishing out the money for RAW is definitely worth it, because Molly was miserable with her constant runny bum, and the other cats hated being around when she used the box (we humans hated being around too, as a matter of fact, lol).
It's just an idea; I had no idea cats could be unable to digest carbs before I got Molly, so I make sure to offer this idea whenever I come across a familiar post like this. If it's not worms or allergies, it could be the inability to digest the carbs.