2.5 yr old cat won't poo in the litter box anymore?

She's been doing this for a little under a year now, but it's been off and on. Now it's totally on. We have two cats and one of them is 100% good about pooing in the litter box. Even when it's a fresh batch, George will poo in the hallway, in front of or near it. Then she'll claw at the ground around it, as if scraping up litter to cover it. She always pees in the litter box, though. We've tried several different kinds of litter, at the moment being Purina Tidy Cats - multiple cats. Any ideas?