Known issue [quote]• Players who have already completed unique missions in the Episode: Echoes story will not be able to launch into them again with other players who have not completed them yet.[/quote] [url]https://www.bungie.net/en/Forums/Post/264270034?sort=0&page=0[/url] As to when they'll ever fix it is TBD. ( I seem to recall a similar issue happening in one of the Year 6 seasons where players were unable to join another player if they had already completed the particular mission that I believe was never fixed)