When we got our kitty...he had diarrhea all the time. Magnus came to us and had diarrhea. We fed Fancy Feast wet food and a mixture of Iams and Royal Canin Persian Formula. We thought his diarrhea was from the stress of a new home and new food since I wasn't able to find that same food the previous owner was feeding him.
He was healthy even tho he went poopies all the time (5-6 times when he'd pooped). We'd run after him and wipe his butt and bathe him to help keep him clean and less stinky.
After a while it was just weird that he didn't ever become regular. I took him the vet once, and that vet de-wormed him and put him on I/D and flagyl too. It worked for a while but it didn't last. The problem came back. I swtiched our kitty to Nutro wet food to see if that would help and it did a little. Took him to a new vet and he was tested for Giardia and later the new vet suspected he had inflammatory bowel disease. What a bummer!!! I thought poor Magnus he's a fairly young cat...we had to give him flagyl on a more consistent basis once to twice a week...just so he'd be regular. The vet told me that it was ok that Magnus diarrhea all the time as long as he was eating, drinking, and active...it's not the most comfortable of situations but it was anything to be overly concerned about.
Later, I heard that it could be food allergy. I had to figure out if he a food allery you'd have to try to narrow it down to the food that was the problem. I stopped giving him the flagyl just to see if it was just a food allergy, and I tried just Iams dry with Nutro wet...no luck. After talking with my fellow cat owners, they'd all tell me how they didn't like Iams cat food and that it was bad. I didn't really understand why but they all said it gave them problems with their kitties. I found this surprising since I thought Iams was a premium cat food. So one day...I I decided to try just Royal Canin for his dry food (a mix of Persian--gold bag and Sensitive Stomach--purple bag) and VIOLA!!! He's all better. Once in a while, he does go twice and it's a bit soft, but it's A HUGE improvement in comparison to the messy bouts he'd have. So sad that my hubby and I would cheer for Magnus when he'd have nice hard poopies. But we are so relieved to see an improvement.
So maybe you can try that Sensitive Stomach formula. I know it's kind of expensive but lots of times I can get a good deal at Petco...buy one bag get one free so after that it's kind of reasonable.
BTW flagyl is terribly bitter...I can even smell the bitterness...I'd given him a suspension form and the soft chew pill form...he didn't like neither. I mixed the suspension with chicken baby food and he'd eat it...and with the soft chew I just pilled him and he took it fine otherwise he'd spit it out and hide under the couch or something.
Sorry this is so long but anyways, good luck and I hope your kitty gets better.