I'd suggest investing in a couple of more litter boxes. My Vixen is very anal about her litter and this developed later. She was fine until she was about 6 months old. Then she started. I have 2 boxes for 2 cats. 2 large covered cat boxes. I need to get another but there is simply no room here for another one. For Vixen one box is for peepee and one for poopie and lord forbid if Lilith does poopie in peepee box or vice versa. I found when I did not have as many boxes as kitties Vixen would have "accidents" as she has a habit of holding until she can't hold anymore. And if Lilith was occupying the box then she just went outside the box. This could be occuring. Maybe possibily there is another cat in the box when they want to use it and they just can't make it to the other.