Many lock-set problems can be corrected before they become so serious that the lock set will not work. Often, a malfunctioning latch assembly or lock mechanism causes the problem. An improperly functioning latch may be the result of a poorly fitting door. The lock mechanism may not work simply because the lock is dirty or dry and needs to be lubricated with graphite (do not use any type of oil).
On mortise lock sets (the type that fit into a large slot in the edge of the door), door knobs 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.