After you have tried the above solution, check to see if the lock operates smoothly with the door open. If it does, the problem is not in the lock, but rather in the alignment of the door and frame. Look for obstructions at the threshold and in the door jamb. Also check the door and frame for twists and warps. The door should completely rest against the door stop and stay there. If the door rests at the top or bottom only, then the door or frame is twisted. If the door springs open then the hinges are probably binding. If you have weather stripping on the door, it could be obstructing the path of the door. Keep in mind, wooden doors and frames will warp and swell with temperature changes and metal doors and frames will twist and rust with age.