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There are a couple of things that I would suggest. Many older cats will simply not use the box because the sides are too high. Arthritic changes might make climbing into and out of the box a challenge. I would switch to a box that has low sides. Baking pans or large pie plates work well in a pinch.
I would also purchase a product called feliway. Use this in all the places where she inappropriately soils. It will help to keep her from returning to that area.
I hope this helps!
Let me know if you still have questions.