i know what you're talking about, my cat is a dsh and has the same problem. it's not all the time, and her poop isn't runny, but sometimes, a bit just stays on her butt. and there have been times where she's sat down on my bed and it's been rubbed off onto my sheets!
the only solution i've really found, as it happened a lot when she was younger as she was not very fond of grooming herself, was to constantly check her butt - after she did her business, when i'd get home from work, just randomly throughout the day, and i'd wipe her if i noticed anything, which she HATED. after a while she learnt that i would only wipe her if i noticed poop on her butt when i checked her, so she started grooming and getting rid of it herself...and when she starts to get lazy again and i notice a smell, i start checking again and she starts grooming herself.
i think maybe you just need to get the kittys into the habit of cleaning their bottoms completely.
although one time, there was quite a bit of poop remaining, and luna started dragging her butt across the floor - leaving poo streaks behind! haha! (glad i have tiles