[Mumeseest]

Glad you decided to try and work with everyone, try not to get discouraged. Here are a few pointers:

1) Make sure the litter boxes are big enough for the cat, you want a box at least twice as long as the cat. Most cat litter boxes are too small, I recommend plastic sweater bins you can get at target.
2) To cats, multiple litter boxes next to each other are one big box. Separate the boxes around the house as best you can to give multiple options
3) If the issue is possibly the litter type, you can try different litters in different boxes and see who shows a preference.
4) Most cats like to see what is going on around them when going to the bathroom, so they do not like covers. Remove them. A nice analogy is thinking about a covered litter box as a port-a-potty. Yuck!
5) For the cat pooping outside the box, check a stool sample with the vet to make sure no intestinal parasites. Also, cats who have arthritis can have trouble positioning themselves in the box for BM or urinating, depends on what joints are affected. Something you can talk to your vet about.

Great website: Indoorpet.osu.edu. Ohio State Vet school Indoor cat initiative