ATTN: All you cat people out there.

Elmo the twenty pound kitten of DOOM keeps pooing on the couch. There are two litter boxes that are cleaned once or twice a day, so we can't figure out why he's doing this. Any suggestions on how to get him to stop?

He started this a while back, then he stopped for a good long time once a litter box was placed in the den, but now he's doing it again. HELP!