Why is my adult cat pooping outside of her litterbox?

Abby is a nine year old cat, and she is a tabby. We have another cat, so we keep two litterboxes, but the other cat does not poop outside his litter Box. Only Abby. At first we thought it was because it wasn’t clean and their wasn’t enough litter, so we dumped both of the boxes out, cleaned and refilled them with clean litter about 3 to 4 inches full. We clean it twice a week as well, but she keeps doing it. We also tried putting tin foil in front of the litterbox and it worked for a while, but then she pooped infront of the foil. She has never done this before, just in the past few weeks. Please help!