Try a small litter box so she/he has no difficulty getting in and out and put it in the spot were she's going (as said above) as she (hopefully) uses it, gradually move it toward the other box until it's next to it. If she's still using it once it's next to the regular box remove it. Also, she has normal stools right? Sometimes not going in the box can mean there's something else going on. If there's soft stool or it doesn't look right get her checked for parasites. Good luck!