@Rubble @jasperwillem Soooooooo, I found out another reason as to why planes don’t leave with all passengers.
I got a notification telling me that a passenger couldn’t board and that I had to clear a pathway for her. When I investigated this incident, I found my passenger stuck in a cell that was already occupied by an information screen. There is also a sofa right in front of the screen. Only the Lord knows how she got herself there.
And after a few minutes, (in gameplay) she teleported herself all the way outside in the parking lot. And by the time she completed security for a second time, boarding had already finished.
Although objects like conveyor belts and information screens do occupy space, passengers and staff do not recognise them and walk straight through them or on top of them 
Have a look at this too if you’re free 