Are you getting the plus when you press right Alt? If so, it’s possible they are popping out to a non existent monitor.
Give this a try.
• Set the sim to “Full Screen”
• Load to the airport
• Pop-out the screen you wish
• Press ALT+ENTER to go to “Windowed” mode, NOTE: do not press anything else or move your mouse
• Press ALT+ENTER again
This should bring the pop-out screen back to your screen, in front of the main window.