My ferret won't use her litterbox?

I got a 3 year old ferret from a rescue shelter not too long ago and it seemed like the first couple weeks I had her she would use her litterbox just fine. I had to move her box around a couple of times cause she was going in different corners. But for a good 2 weeks or so she was using it just fine. I kept on top of it by cleaning it every few days and made sure to maybe leave one or two stools in the new pellets so she knew to go there. Then for no reason what so ever the following week she would poop on every other level of her condo. So I cleaned the cage really well after the third time she did it to get rid of any lingering scents. I finally ran out of pine pellets and switched to Purina's "Yesterdays News" Paper Pellets thinking maybe she just didn't like the last brand i bought. She still won't use her litterbox. She stopped pooping on the other levels of her cage, but she still won't go in her box. She will poop and pee right in front of it, or move it and go behind it. I constantly clean it up and try to deoderize that area afterwards and place her stools in the box, but she will not use it. she is starting to sleep in her litterbox now too even with stool in it. What the heck is up with her? She did so good when I first got her home.