How to Fix A Door Latch That Will Not Engage
From DIYinfo.org
If a door sags a little or becomes slightly swollen, the latch bolt will be out of alignment with the striking plate.
A small misalignment can be corrected by enlarging the cutout in the striking plate with a file.
Otherwise, remove the striking plate and re-fix it in the correct position.
If the plate has to be moved only a small distance, plug the old screw holes with small wooden pegs and drill new pilot holes for the fixing screws.