It doesn't always work for old entries: you tick the boxes, get the message but if you edit the entry again, the boxes are not ticked.

When you say 'get the message', do you mean the initial 'message sent to crossposter' message, or the success message in the Inbox? There's not yet a setting to indicate 'crosspost pending' (though there probably should be), so if you checked before the crosspost finished, it wouldn't show up yet.

If you checked after the crosspost finished, then it's probably just a bug. :)