Cats stopped covering their poop and pee

My two kittens male and female, (both 6 mo) have stopped covering their pee n poo recently.

My girl cat did occasionally leave it uncovered but her brother would come in and cover it.

Sometimes when even passing through the litterbox he'd go in and just cover the already covered poo.

Recently they both have stopped covering it. First i thought it was because there wasn't enough litter maybe but today I added a lot more than usual but they still haven't covered it.

I only got them 2 months back, they have been dewormed, vaccinated and sterilized.