A sticking door latch might require some lubrication, or it might be jamming against the strike plate on the door jamb. You should first test the door latch to ensure the latch is sticking inside the door and not due to a misaligned strike plate. Turn the knob to open the door and release the knob slowly. Once you let go of the knob, grab the end of the latch bolt with your fingers and pull. If the latch bolt doesn't move and is fully extended, the issue is with the strike plate. If the latch bolt moves, it is sticking in the door knob and requires some disassembly to fix.
Remove the two screws on the inside door knob faceplate with a Phillips screwdriver. Hold the front door knob with one hand and the inside door knob with your other hand. Pull both door knobs away from the door.
Spray a couple of shots of spray graphite on the latch mechanism where you see a seam. There are seams on the latch sleeve and in the hole in the center of the latch.
Place the front door knob assembly into the front of the door, ensuring the center shaft of the knob inserts through the hole in the latch mechanism and the two threaded sleeves fit into the two small holes on each side of the center hole on the latch mechanism.
Insert the inside door handle onto the other side of the door. Thread the retaining screws through the mounting holes on the faceplate of the inside door knob and into the sleeves of the front door knob. Tighten the screws with a Phillips screwdriver.
Turn the door knob so that the latch bolt retracts into the door and hold it for a moment. 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.