I understand this might be a bit of a stupid idea but i think it would be usefull to help to ease the confusion. Make it so that you can click one of the landing slots in the picture below and tbat it specifially tells you, missing arrival slota for (destination, other destinations already landed) or something like that
Edit: secondly same thing for when planes cant pushback such as in the picture below. For ex. When you click a message appers and says missing departure slots, 2 already filled.