If a door won’t latch closed unless you slam it, the strike plate mounted on the door jamb is probably out of alignment.
Instead of simply moving the strike plate up or down, try this trick: Remove the strike plate and clamp it in a vise, making sure to pad the jaws with cloth to protect the finish. Then file the interfering edge of the plate to enlarge its opening slightly so the latch bolt can enter. Reinstall the strike plate and test the door.