Locked Pages docs won't stay locked

Each time I lock an existing Pages doc in the window header, the lock tick box is ticked for about a second then un-ticks itself. I can create a new doc and have it stay locked but not existing ones. I’m trying to make sure I don’t accidentally edit/change a particular document.

Any ideas how I can make this happen?

MacOS 10.15.4 (19E287)

Have you tried locking it from Finder?

Is it on a drive/folder that can’t be written to (in other words, the lock status can’t be set on the file because the file is on a locked drive)?

Not that I can see JohnAlt. They are saved to a folder on my MacBook Pro which is linked to Dropbox - might be an issue with Dropbox?

Yep! That’s exactly what it is. Dropbox doesn’t support some extended attributes.
The workaround is to make it Read only in the permissions. (Right-click, get info, sharing & permissions)
cannot lock

Many thanks. That’ll do the trick.

One curious observation: A document I had been trying (and failing) to lock had the little padlock against the pages icon in the document’s window and yet, the lock tick box was of course un-ticked. It’s gone now (after toggling the tick box a few times) and needless to say I can’t re-create the issue.

Thanks for your help though.