This happens, inter alia, when the ticket hasn't been fully ticketed, also for non BA bookings, or booking that have been altered online once already. If done online this can happen immediately, but sometimes not, especially if there were system updates overnight and ticketing gets queued. Ironically if you left it a bit longer you may be able to do it online. But I guess it is annoying, especially if your service centre works restricted hours..... However if you are closing in on the last 24 hours before departure you need to contact whichever service centre is open.