litter box

When we got the new kitten she seemed very mean and isolated(i have 2 other kittys) pretty much sunny(new kitty) stayed on the couch and wouldnt move so we put an extra litterbox beside the couch for her to make it close,now she is doing her buisiness behind the couch and in the litterbox.
How do i get her to stop going behind the couch and strickly in the litterbox?i clean behind the couch daily(about 3times).She is alot more comfy in the house now and follows and plays with the others kittys and me and my man nicely but still chooses to go behind the couch,iam confused now :-(

What do you use as cleaning agent? You need an enzyme base cleanser to remove all traces of waste odor.
Also, where is the litterbox now? Is it still near the couch or somewhere else? Make sure your cat knows where it is.