I need help, please!
I'm new to chickens and my 15 chicks are 6.5 weeks old. 6 of them are Marans.
My sweet Cuckoo Marans "Dot" (yes, I know, you are so amazed by the originality of the name! LOL) has always had a bit of a poppy vent area. When she was tiny I would clean it off almost daily because I was afraid it was pasty butt and would block her vent. I've watched her poop and she hasn't ever had trouble, but she is always dirty around her vent. I've cleaned it repeatedly (like once every week or so) by gently soaking her back end in warm water and then loosening any hardened bits, BUT it keeps getting yucky. She stinks.

Last time I cleaned it I trimmed some of the feathers thinking that maybe they were in the way, but, honestly, I have other chicks with much fluffier butts and they're very clean. She tries to clean it, but can't reach all the way. Her wings are so long that they cover her vent area when walking around, but she holds them to the sides to poop so they aren't the problem...
ETA: None of the others have this issue. Their water has ACV in it and I give them kefir or yogurt every couple of days. They also eat a whole grain, soy-free feed: