Patio door locks may freeze or not work from time to time. Element exposure as well as a build-up of dirt and debris can cause locks to malfunction. Overuse may misalign the lock and it will not catch or bolt correctly. In some instances, the lock won't catch because of dirt build-up on the tracks of a sliding patio door. The build-up keeps the door from shutting tightly and the lock will not catch.
Check the patio door's closed position. If it's a sliding patio door, inspect the tracks for obstructions like dirt build-up. Brush away the dirt with a broom and clean the dislodged dirt with a dustpan or vacuum. Grease the tracks with a lubricant for better sliding operation and to close the door snug to the frame for the lock to catch.
Locate the latch adjuster screw near the lock's latch mechanism. Turn the latch adjuster screw clockwise with a screwdriver to make the latch protrude further from the lock's recess or counterclockwise to make the latch sink into the latch recess.
Close the sliding patio door and try the lock. Lock and unlock the patio door to see if it operates correctly. Make adjustments to the latch adjuster screw as necessary until the latch catches properly and locks the patio door.
Turn the patio door knob up and down to inspect it for foreign objects, dirt or rust. Clean the door knob and lock with a degreaser and a wire brush. Spray lubricant onto the entire knob and lock assembly for better operation. Try to lock the patio door.
Free the latch. In the instance the latch is not protruding and retracting, it may be frozen into place. Open the door and place a flathead screwdriver over the top of the latch. Tap the screwdriver with a hammer a couple of times with enough force to dislodge the latch but not break the mechanism. Repeat underneath the latch to free it completely.
Replace the patio door lock. If either fix does not make the patio door lock correctly or at all, remove the door lock with a screwdriver and fit a replacement lock into the recess. Take out the existing lock and bring it to a home improvement or hardware store and purchase a new lock that will fit your patio door.