I had to reach back and push the rudder down with my hand for it to work. I tried several times to lift it back up and put it back down, with the steering handle in several positions, but nothing worked except manually reaching back and pushing it down.
Mine is sometimes tough to lock as well. After you pull the rudder down and tension the down line
in its locking cleat, put a little speed on the boat and cycle the rudder control full left and full right. This should snap it into the locked position.