It’s easiest to develop good litter box habits in rabbits by limiting their space at first. Use a playpen to confine your rabbit to one area, even if you intend to give him/her free reign of your home eventually. This allows your bunny to get acclimated to the area in the beginning. Once your bunny consistently uses the litter box, you can gradually expand the area. If your rabbit starts “forgetting” to use the litter box, then limit the space again until good habits resume.
Most often it takes our babies about 3 days to “readjust” to their new home in regards to litter box training. Keep in mind that it is a new area and a new litter box so there will be adjustments. Just be consistent and add any mess back to the litter box. If they remain stubborn that they prefer to go to a different area, move the litter box there.