Cat going #2 outside of litter box?

We have 2 cats, both female, one is 3 and the other is 2. We have had them both together for almost 2 years. One of the cats has recently (for the past 6 months or so) started going #2 right outside of the litterbox. She pees inside of the box and the other one always uses the box. My husband and I have tried... show more We have 2 cats, both female, one is 3 and the other is 2. We have had them both together for almost 2 years. One of the cats has recently (for the past 6 months or so) started going #2 right outside of the litterbox. She pees inside of the box and the other one always uses the box. My husband and I have tried everything we can think of and we cant figure out why she is doing this. We keep the box clean, scoop it every day. We have the biggest box we could find, it is a round box with dome shape lid. Any suggesstions on why she does this and what we can do to make her stop? Why only recently?
Update: Just to clarify, the cat pees in the litter box, only goes #2 outside of it.
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