The BA booking system, Amadeus, purges flown bookings about 24 hours after the last segment is flown. Miscellaneous segments can be added to prolong the life of a PNR, but if your PNR is purged, it can take longer to recall it/obtain the necessary details. It's not the same as reprinting an old invoice from a billing system - because there is no invoice per se, just your old PNR and used e-ticket, which may or may not be immediately visible...