For serious lock problems, it is usually best to call a locksmith or to replace the lock entirely. If your problems require professional help, keep in mind that removing the lock and taking it to a locksmith is far less expensive than having the locksmith come to you.

If a door latch does not operate smoothly, the latch bolt on the door may not be lined up properly with the strike plate on the doorjamb. Repairs range from making minor latch adjustments to repositioning the door.

If the latch does not catch, close the door slowly to watch how the latch bolt meets the strike plate.

The bolt may be positioned above, below, or to one side of the strike plate. (Scars on the strike plate show where it is misaligned.) It is also possible the door has shrunk and the latch no longer reaches the strike plate. Once you have figured out the problem, try one of the methods shown here.

On mortise lock sets (the type that fit into a large slot in the edge of the door), doorknobs may become loose over time. To tighten them, loosen the setscrew on the knob's shank. Hold the knob on the other side of the door, and turn the loose knob clockwise until it fits snugly.

Then tighten the screw until you feel it resting against the flat side of the spindle. The knob should turn freely. If this does not help, remove the knob and check the spindle; if the spindle is worn, it must be replaced. If the whole lock set is worn, it is best to replace it entirely.