They might just not like your cat litter of choice. One of my friends tried switching their cat litter brands and one of the cats didn't like it. The cat let them know this by jumping up on the coffee table in front of them while they were watching TV and took a crap in front of them. No problems at all since switching back.

Other problems could be a bladder infection. The cats learn to associate the pain of going to the bathroom with the litterbox. There could be lots of reasons for them to avoid the litterbox.