If a bunny produces excess cecals that get stuck to the rear and perhaps get fecals stuck in them too, the rabbit won’t want to eat them and then you get a build-up of sticky-poop. A healthier diet takes a bit of time to take effect, because the intestine, esp the cecum, needs to develop a microbial flora that can process all the roughage properly and extract all nutrients out of this more fiber-rich food, and that doesn’t happen overnight.
I have one bun who can be very sloppy with his downstairs cleaning, so I always have to check him to see if there’s sth I have to pick out, sometimes even butt-bathe off. He’s not fat and he’s not geriatric although he’s getting on in years now, he always had this problem though. I’ve fairly recently started giving him a probiotic that has e faecium (nasty name but still good bacteria) in it, same as is in f ex Benebac, and it has really helped.
Edit: Keep up the better diet + possibly a probiotic suitable for bunnies, if problem lingers you can have her poop checked for parasites. Here is an article from the HRS on Intermittent soft Cecotropes, the medical term for this condition. As you’ll find, a change in diet is the first action to take: ISC rabbits