How can I deter my cat from pooing outside the litter tray?

I have had my cat from her being 6 weeks old. She was litter trained by her mother when I got her and continued to use a litter tray successfully for quite a while, recently she has started pooing behind my bed and behind the couch and yesterday beside my wardrobe!!! she still uses the litter tray regularly and has a cat flap, although I'm sure she comes in to toilet rather than do it outside, but this behaviour is becoming really annoying now. She knows it is wrong because i've told her off for it and when i catch her now she runs off and hides. I've heard the deter sprays are not much use although I've been reccommended to use a citronella spray. Can anyone shed some light on this and how i can stop her doing it. I also have an adult cat who never does this and always uses the tray.