Kitten won't use her litter box?

I recently got a kitten. She's been at my house for about a week and a half and she is 16 weeks now. The first night we got her we had to cage her because my dog was trying to attack her and we needed to set up a room for her to stay in. Since there was no litter in the cage she pooped in the cage. We cleaned her and moved her into the room we had prepared for her and she continued pooping in her bed instead of the litter box. She pees in the box but will not poop. We got the dog to like her and now she roams around the entire house. Now that she has more space she won't use the little box at all. She has used my moms potted plants, drapes and carpets as a bathroom and now my mom wants to send her back to the pet store. I love her so much mad really don't want to get rid of her. How can I get her to start using the box? We have tried moving it, uncovering it, covering it and a litter attractant. I really want to keep her! What should I do?