The people in the check in desks won’t leave:
I know it might sound stupid and I am only new to the game but they are all crowded round the check in desks with red speech bubbles over.
Staff or passengers?
If it’s passengers, they can’t find a route to the next area (open security).
If it’s staff, they can’t get to a staff room or their next job.
What symbol is in the red speech bubbles? Can you post a screenshot?