How to make litter box less stinky? Sometimes she doesn't bury (It's clean...)?

We have three cats that use one litterbox and we have no problem with the smell. We use Fresh Step multi cat for litter and for food they eat Taste of the Wild. Purina is crap in a bag. I would look into high quality foods that don't have corn, wheat, soy, by-products, or grains. When the cats were on Purina food they started shedding more and their poo stunk a lot worse amoung other things.