A league date's first generation uses only players opted in for that date. If fewer than 4 are opted in, it uses everyone you have not sat out instead. Regenerating a date uses each player's group, minus anyone not playing that date.
Check the player's opt-in status for that date on the Players tab.
Open Admin > Edit Player Groups from that date. If they are under Not playing, drag them into a group.
Regenerate the date.
See League opt-in and Generate matches by play date in a Rotating Partner Round Robin league.