Mail 'From' issue

I'm temporarily using anothe macbook whist mine is in the Apple store being fixed.

I've reluctantly set up mail, and have added three accounts (I've a separate user account on this mac).

When I try to send mail, the 'From' drop down only shows two of the three accounts that I've added. They're all IMAP, all added in the same way, and all use the same outbound smtp server.

I haven't a clue how to look for a solution to this, all the search terms I've tried don't lead to anything remotely useful :-)