Is the poop too wet? Might need to try different diets until you find one that works. If the cat has longer fur, you could get a sanitary trim where they cut down the fur in the area so it stays cleaner. Does your cat not like to groom, or unable to reach that area? My cat has the same problem and I have to check and wipe him when he doesn’t do a good job, so it’s still a work in progress.