My girl had the same issue with poops on her butt. They would form a large like patty just below her tail. I did the same thing you did to get it off, it comes off pretty easily. Then I started reading more about "poopy butt" and found it could actually be related to her diet. It's not that they can't clean there, it has to do with her digestive tract. Rabbits are all different, what is good for one might not be for another. I found out she had a sensitive stomach. I completely took her off pelts for a month (only hay, no greens, pellets or treats) and the "poopy butt" completely went away. Then I found the best pellets I could buy and started her slowly on those. She never got "poopy butt" again after that. I started really monitoring her, introducing foods slowly back into her diet. If she got poopy butt I then knew what was causing it.
She never showed any signs of being sick or unhappy while she had her poopy butt, she was just as active and sweet as when she didn't. So, I wouldn't have known anything was wrong. And, it wasn't that it was "wrong" it was just she was more sensitive to foods then her bonded mate was.
In the end the poopy butt went away, no more washing her. A relief to both of us.