How can I fix my multi point door lock?

The actual mulitpoint isn't a problem, but whereas I used to have to turn the handle up as far as possible to lock the door with the key, now it only goes halfway up and won't let the key turn a full revolution to lock the door. The handle will turn down fully as it always has done. How can I fix this? I've tried the obvious WD40 and have taken what I can apart to look for any broken bits or blockages etc, but nothing seems to make sense as to why it's stopped working! Cheers, James.