The problem is that the flight was international. While many pilot groups are D6U on domestic flights, those few airlines that are approved for D6 on international flights are all D6L, allowing only one jumpseater per available jumpseat, no matter how many cabin seats are available. Flight attendants, who are categorized as D7, are not authorized to travel D7 on international flights at all, so you should have gotten the seat ahead of her, assuming you are on the approved list of carriers for D6 international.