One of my cats had a problem with runny poo when he was a kitten. The vet checked him, said it was worms and sent him home with meds. At the end of meds, he still had the problem. She checked him, said he still had worms and it wasn't uncommon, he came home with meds again. After the meds, he still had problem. She said his worms were gone, so she was checking him for another internal parasite he may have picked up but all tests came back negative. She ended up diagnosing him with IBS (irritable bowel syndrome). During that time, a few people also informed me that he was a Norwegian Forest cat and I knew he had been living outside on a farm for his 12 weeks before I got him.
I realized his diet may have consisted of what he could catch--mice, etc. I decided to put him on a grain-free diet and see how that went. It totally cleared up the problem. He's two now and if I give him any food with grain (wheat gluten, rice, etc.) he has the runny poo problem again.
The vet approved the grain-free diet for him and my other cats are on it as well and doing great. If one checks around, the prices aren't any more than the foods with grain in them. I divide the ounces in a can by the price (including tax and/or shipping) to get the true price and keep a notebook of the food my cats like and where to get them. I love ordering on-line from pet food direct and you can earn money back from FC if you use the toolbar or go through the FC shopping mall (unless you're using one of the pet food directs special offers). They have times when it's free shipping on orders over $49---so then it pays to stock up. The cheapest prices food is a brand called "By Nature" and my cats love it. Other favorites have been the pork food, rabbit, they don't care for lamb, Weruva is a big hit but kind of pricey.
They also get canned pumpkin a few times a week, plain Greek yogurt a few times a week (just a little tsp. of either, never both in the same day), a fish oil capsule split between them daily, and I plan to start them on a partial "raw" diet soon ("Primal" food can be ordered from on-line from pet food direct, it's pricey but he boys are worth it.)
So if you've tried everything else and it's not working, and the vet doesn't know why....try going "grain-free". PetSmart, Petco carry grain-free diets. Even Walmart has the lowest prices I've found on "Fancy Feast"---if you get the cans that are labeled "Classic" (turkey & gibblets in a olive colored can, chicken in a red can, etc.) are grain-free (check the label to make sure it's gluten-free but the ones I've found that are "Classic" have no wheat gluten). Also if your Walmart has a refrigerated pet food, the bag for cats is grain free and my cats LOVE that. It's under $5 a bag.
Most of the pet food recalls in the past have been because of rice or wheat. Going grain-free also eliminates that problem. Hope your kitty is better soon. I know how frustrating it can be dealing with that problem week after week. Wish you both a lifetime of happiness!!