While it could be some problem with the cooking mechanism, sounds like you may have a broken main spring. Take the locks off carefully and have a look. If anything is broken, beware of flying or falling parts.
Matt's advice is good. If you feel uncomfortable taking the locks off, then remove barrels and fore-end and grasp the left cocking lever where it protrudes from the action knuckle and see and note how it moves up and down as you move it; then do the same for the right cocking lever. This will give you a clue of what the left lock mainspring condition is as well as if the left side cocking lever is broken. Sometimes the mainspring swivel will break. Not often a cocking lever breaks.
