Well it is part of a DUB-Hawaii-DUB itinerary with a couple of BA flights and shows up on the ITA Matrix, but not on the travel agent's system.

My only guess right now is that the travel agent searched for this specific flight without the system realizing that it is part of this itinerary and thus couldn't find the BA flight number.

Will probably try a different travel agent next week, see if they can do better...