Hi when you try to lock the coupler is your engine powered up if not the coupler is defective, and you need a new one! there not that expensive! Is it a ps 3 locomotive? you could contact MTH, and they possibly would send you a new coupler! you could contact stockyard express could fix it for you there from Ohio!
here is the coupler part number but is not in stock maybe call mth and they'll have a substitute part number!
Alan