What is a latch bolt?
Keeping this in view, what is the difference between a lock and a latch?
A latch fastens a door, gate or window in position but does not provide security. (See Wikipedia.) A lock prevents anyone without a key from opening a door/gate/etc. A latch keeps something closed but does not lock it.
Then, what is a deadlocking latch?
Deadlocking latch bolt (deadlatch) is an elaboration on the latch bolt which includes a guardbolt to prevent “shimming” or “jimmying” of the latch bolt. When the door is closed, the latch bolt and guardbolt are retracted together, and the door closes normally, with the latch bolt entering the strike plate.
Spray a shot of graphite around the inside edge of the latch bolt hold. Turn the door knob several times back and forth to distribute the lubricant. Wipe off any excess with a rag. Adjust the strike plate up or down as necessary if the latch is simply sticking in the strike plate.