ss wrote
Rod Speed wrote

But the; pins wont necessarily slide freely.

If the key slides in then the pins will have been lined up to the shear
line.

Not if the pins done slide freely enough so that the pin comes down
to the key. That why powdered graphite works, it ensures that happens.

Taking in to account the spare keys dont operate the the lock then I would
guess one or more of the pins have been worn or damaged.

Much more likely that the pin isnt moving freely down to the key.

I just had my front door key which has been used multiple times
a day for more than 50 years now, end up very hard to get the key
to open the door. Quirt of powdered graphite fixed that completely
and now the key turns so freely that you wonder if its broken.

Other possibility is the barrel has seized

Never seen that happen, even with the padlock used to chain the
trailer to the house which has a very hard life indeed. Corse its a
decent quality brass heavy duty padlock.