You need to make sure that auto enlarge is not checked, you will find this from the config menu then general config.
The reason that happens is when I was making the screen I had a background image then deleted it but for some reason it deleted from all screens except one and on that one it enlarged itself. I just cant delete that and the only way I could fix it would be to redo the whole screenset and I just dont have the time at the moment.
Hood