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Maybe take her to the vet see what the vet will do. If nothing then maybe have her locked in a room and leave her there for 1 hour and don't check on her and when the 1 hour is up then see if she has...

How about a toy poodle or a jack russel. They are good.
Lluv I won't to be a vet:D

Well she could have a medical problem or maybe you should put her with the litter box and close the door(best place laundry) and stay outside for about 10 minutes and see if she has used the litter...