The door that worked yesterday and not today
After a wet week, an original timber door swells, lifts against its frame and stops aligning with the keep. The key turns, the bolt throws into nothing, and eventually the door will not open at all.
This is a carpentry problem masquerading as a lock problem, and replacing the lock would fix nothing. We open it, then ease the door, adjust the keep and service the mechanism so the seasonal movement stops jamming it. It is the cheapest kind of Bo-Kaap job and the one most often misdiagnosed.