One of our cats won't poop in the litter box, why does he do this and how can we stop it?

We have our older male cat who is about two years old and this summer we got a male kitten. One of them, we don't kow which one, won't go poop in the litter box but will go out side of the litter box a few feet away from it infact. Why are they doing this and how can we get them both to go in the litter box?