Hi there,
Is Coco just pooping outside of the litter box or peeing as well? And is it a pile of poops or just scattered poop here and there? Both my rabbits are messy as well and despite both being litter trained they will leave a few scattered poops here and there. The scattered poops are territorial poops and you are fighting a losing battle there trying to get your rabbit to poop only in the litter box especially if she is a messy bun like mine are. I am fine with the few oops as long as they do not pee outside the litter box and they don’t.
If you are finding a pile of poop, then that is definitely a litter box problem. For that, just make you either have a hay rack above the litter box or clean hay in the litter box because rabbit like to eat and poop at the same time. Also you could try cleaning thoroughly, the places that she pooped and peed, since if those places even smell remotely like pee/poop she will want to go there instead of the litter box. And finally,keep the litter box as clean as possible, especially if your rabbits are bonded and live together because the litter box gets dirty faster and nobody wants to go to the bathroom in a dirty toilet.
Hope this helps! Good luck!