Keyboard event is not activated

hi All,

I m creating a project of maze game in which have to use keyboard event.
All the coding is done in as3 and i m using swc files in my project.
When i published the swf, it shows me intro screen then it goes to game screen when i click on play button. Then the game screen gets loaded (i did addChild for this as i m using swc for each screen). Now the keyboard event should capture when any key is pressed.
For this i wrote :
Main.STAGE.addEventListener(KeyboardEvent.KEY_DOWN , pressSpaceBar);
Main.STAGE is a static variable stored in another .as file.

But the event is not firing on loading the game screen. It fires when i click anywhere on screen(screen selection).

Why this is happening ? and why the keyboard event is not firing ?
can somebody plz help me out ?