All locks require regular maintenance, and if yours is not in good working order, it may be time for you to clean and lubricate it. Push-button knobs don't have many working parts, so maintenance is minimal but still necessary. If you lock is sticking or turning hard, it may be due to buildup within the lock, excess moisture or little lubrication. If the lock seems to engage but the door doesn't latch anymore, the strike plate may be loose.
Open the door and remove all trim screws from the knob and door edge with a Phillips screwdriver. If there are no screws on the knob trim check the trim edge for a button or flat head screw or button. Remove the screw or press the button with a flat head screwdriver or the tip of a Phillips head screwdriver and remove the knob trim.
Pull the knobs away from the door. Locate a small push button on the interior of the knob and press it with the end of a screwdriver to release the button.
Clean all the pieces of the knob with a rag, then spray them with a silicone-based lubricant. Reassemble the spring, button and doorknob.
Reinstall the knob and test it with the key. Tighten the strike plate screws on the door frame opposite the knob with a Phillips screwdriver.