To follow up on what others have said, two things immediately come to mind: she doesn't like where her litter box is, or she can't smell it because it's too clean.
Not only should the box be somewhere that is quiet and out of the way, but she should also feel like she has multiple escape routes. Unfortunately where we humans like to "stash" the box is generally the worst place, such as in a corner or between a piece of furniture and the wall. Any outside cat would be especially sensitive to this.
My own BooBoo Kitty has box issues as well; it was especially problematic when first my oldest cat, and then my oldest dog, started having issues with incontinence. If someone made a smell, by golly Boo was going to cover it! He especially liked any bedding they used or anyplace where they would lay for any length of time.
Both Lucy and Baxter (the elders) have passed now but the next two elders, Skeezix and Sadie, are about the same age now. We've been fortunate with them so far, but I still never leave pet bedding, blankets or any type of cloth where Boo could get it.
Cats are interesting creatures, aren't they? But I love the little monsters, oh yes I do.