cat has been using the litter box regularly but also pooping outside it

just as the title says. i’ve been using the same pine pellets that she’d been used to at the shelter and am planning to transition her to pretty litter. she’s been mostly good about using it, but twice she’s pooped outside of the box. the first time was a couple days after she was adopted so i figured it must have been her struggling to adjust, but last night she did it again after a few days of using the box. the box is cleaned once a day. what could be causing this behavior?