Even a simple itinerary may be a problem. I deliberately chose a departure date when it showed (and is still showing) two "A" seats available on MRU-LHR (using Expert Flyer) but two attempts to have BA-MRU book it have been fruitless-- although we are waitlisted. (Also ITA also shows two A seats as being bookable). If BA-MRU can't book them, who can?