For DL-operated flights departing from AMS or CDG, it can often be a little wonky since the flights are handled by KL & AF, respectively. Sometimes I've been able to get a DL BP in the app and sometimes I haven't - no real rhyme or reason. Therefore, not surprised to hear you ran into an issue, but there's no real solve as far as I know other than to give it a try and see if it works; otherwise, if not, to stop at the counter at the airport.