I had a locksmith make a duplicate key of my mailbox lock, and it didn’t work (didn’t open the lock).
The cutting looked good, I saw no difference with the original key.
Until I noticed this notch:
Turns out this notch accepts this tiny “bump” on the cylinder:
This notch was missing on the duplicate. I milled a notch and now the duplicate key opens the lock:
Article Link: Quickpost: Fixing A Duplicate Key | Didier Stevens