My cat Taco is a Manx and when I first got him, he was about 8 wks old. I didn't know much about cats then, and I was giving him milk and cheap dry food. He often had a messy bum from loose-ish stools. I finally stopped the milk. This continued. I spent hundreds of dollars, I'm sure, taking him to various vets - first having them test his stool for parasites of fungus or some other cause (you should consider doing this) of the problem, then I put him on a dry food (a prescription food) that was high in fiber. It helped somewhat. I also had him on a 2-3 month course of Prednisone (which I don't really recommend; vets back then were just sort of playing trial and error). They tried him on Metronidazole (Flagyl) antibiotic. We tried it all. It's something I just resigned myself to living with and I was often cleaning up his butt. Finally I realized what the problem was; he's allergic (or has a real intolerance) to grains! Wheat, oats, barley, rice.............so once I figured this out, what a world of difference.
I just looked up the food you feed - is it the kitten or adult and is it the dry?
if it's the dry, at least for the adult, it contains a lot of grains: barley, rice, oatmeal, millet. It's possible that THIS is what's causing the problem?
Luckily nowadays they do sell various dry foods that don't contain grains at all, which is great because cats are obligate carnivores (meat eaters) so in the wild it's not like they'd eat grains anyway, and for some it's harder to digest. If you do a search on Google for: cat food dry grain free, you'll find the names of some good premium foods: Orijen, Go Natural, Now (unfortunately I think these are all made in Canada so not sure how easy it would be for you to get them if you're in the U.S. However, I just went to the Now food website and you can search for a store locator, whether in Canada or the U.S. so I'm betting that's the case with the others, too), I know there's also EVO that you should easily be able to get in the U.S.
I'd still recommend you take kitty to the Vet for a good check-up and take a fresh poop sample (put it into a ziploc bag) in with you, so they can analyze it.
I know for many years, the Vets just pushed us to use prescription foods, mostly Hill's and Science Diet (which frankly, I think they're crap and they're filled with fillers and corn and grains and junk)...which certainly didn't help Taco's situation at all.
Taco's diabetic and 15.5 yrs old now; he no longer gets dry food because it's easier to regulate his diabetes with low carb canned food.....but the odd time I give him a bit of dry as a treat, it'll loosen things up a bit...though I've started using the Go Natural now (for my other cats) and when he eats that (grain free), no problems with stools.
TO ADD: for many years I would be on pins and needles when visitors would come over because Taco was notorious for leaving a "turd" behind. It was like that last bit just sorta hung on and if he'd jump up on something and then jump down, it would fall to the floor. I still deal with this a little bit, it's just a Manx thing I believe......but at least now his stools are nice and firm and it's easier to pick up than back when they were loose or very soft.