So, cats defecate outside the litter box for a number of reasons. Sometimes just one bad experience with the box is enough to change their habits for the rest of their life (cats with cystitis comes to mind in particular). There are several things you can try, but I would probably only do one at a time, that way you know which one works.

The first thing I would do is set up another box in another location if you have one you can put a box in. Make sure your kitty is using the new box and then you can get rid of the old one altogether. Sometimes just the location and the memory associated with that is enough to make cats eliminate outside the box. If you don't have another location, I would try moving the box just a little bit nearby where it is - sometimes just a reorientation is enough.

You can also try a new litter. Gradually mix in the new litter with the old litter until it's all new litter. (I'd do this over a couple of weeks). Another thing cats get weird about.

The last thing you might try is to closely examine everything in the area where you cat box is already placed. Is it near a dryer where the buzzer startles your kitty when she is using the box? Is there a lot of traffic with people coming and going? Sometimes just little things that would disturb your kitty on the box is enough for them to "make a statement" by eliminating outside of it.

I know when my cat is stressed or upset because he will pee in my sink. I move the box for a few days, try to keep everything as low key as possible, and he will go back and be happy after that.

HTH.