Starting a mission before it leaves the mission list seems to also make it so quickplay can not match players into it.
Not sure if there are other factors causing matchmaking to not find any players but this is a common issue for me.
Waiting for an hour with no players being put in my game besides 1 that possibly joined through the social menu:
https://www.twitch.tv/videos/1675311066
Waiting for 5 minutes for 1 player, having to invite someone at 10 minutes, and nobody else being added to the mission. Had to play the rest of it with a bot:
https://www.twitch.tv/videos/1676571695