Instructions
Restart "Flight Simulator X." Files and glitches can occur within the game for a variety of reasons. If one occurs, typically restarting the game and reloading your flight information will fix any errors with the ATC window.
Click the "Push to Talk" option from the "Voice" options when flying with other pilots. One common error people have with the ATC window when doing multiplayer flights is that they cannot properly communicate with the other pilots. Clicking the "Push to Talk" button allows you to assign a specific key that needs to be pressed anytime you want to relay information to the window.
Reinstall any add-on packs, such as Radar Contact. These packs add new components that can sometimes glitch or interfere with the game's interface, including the air traffic control window. If the add-on is not communicating with the game clearly, you may have to remove the add-on altogether from your game.