A couple of years ago, I fitted new anti-snap, anti-bump eurocylinders to the locks on our front and back doors, keyed alike so I didn't have to carry around two different keys. So far, so good; all worked well, key turned both locks from inside and from outside.
The back door is seldom used. When I came to try it a few days ago, although everything was OK from the inside, when I tried to put the key in from the outside, it wouldn't go in more than a few mm. I tried gently tapping the key, and then squirting some PTFE WD40 to free it, but to no effect. It still won't take the key.
I did wonder whether some oik had put something into the lock, but why do the one door and not the other, as the front door lock is perfectly OK.
Any suggestions on freeing it up?