peeing outside the litterbox

my cat has been peeing outside of the litterbox again. yes, she has been to the vet, and they found nothing wrong. with that, this isn't new. she started to pee in our basement after my dog got sick and was having accidents down there. i moved her litterbox to the basement, literally inches away from the spot they both seemed to use (can't put it directly over, as it is in a hallway) and we had the carpet professionally cleaned which solved the problem then.

now, two years later, she has started again. she has two XL litterboxes in the basement by the 'spot' (she is only 6lbs, so this is plenty of room), cleaned daily, no litter or food switches within the last year, she is still peeing and pooping in the litterboxes, has a clean bill of health, has been the only animal in the house for two years (she isn't cat friendly, so getting her a buddy isn't able to happen), i play with her for at least 30 minutes everyday, and there has been no lifestyle/house changes.

i have cat pheromone diffusers in the basement and she also has a pheromone collar on (both are fragrance free)

any suggestions on what i should do?