Why is my mom's cat pooping in the floor?

My mom and dad have 2 female white cats that are 1 1/2 years old. They are both litter box trained and share a box, and have been sharing one just fine since my parents got them. The litter box is located in the laundry room, and is well away from their cat food, which is located in the kitchen. The cat box is hand-scooped multiple times during the day, so it is very clean. They moved into a new house a couple of months ago (which is much larger than their old house and apartment), but one of the cats keeps pooping on the dining room carpet, located in the other room. We've tried shampooing the carpet, spraying it with citrus air freshener (My dad heard cats don't like the smell of citrus), and even laying down moth balls in plastic bags. We've also tried the "rub her nose in it and spank her" technique. She started pooping in the floor a couple of weeks ago, and the moth balls worked to deter her for a little bit (about 2 days). Today she was lingering around the dining room, and my dad picked her up and put her in her litter box when he noticed her starting to act like she was squatting. Then, next thing we know, she walks back in the dining room and pooped in the floor again when our backs were turned. It's never in the same spot, but is always in the dining room, and she pees in the litter box just fine. What is going on? How do we stop this? She doesn't cry or act as if she is sick or in pain beforehand or after and the other cat does both "duties" in the box just fine as well. They have been using the same brand of litter since they've had them.