If you do multiple tasks for a mission in the same session (without saving and exiting), it's possible that the subsequent activities won't count towards the mission. For example, this includes multiple group rides or workouts in the same session.
We're working to fix this in an upcoming update.
In order to avoid your efforts going to waste, we recommend starting a new ride after completing one of the requirements for a mission.
If you're on iOS, we also recommend force closing the Zwift app before starting a new session. Instructions for this are on Apple's website.