Flash movies can’t start in full-screen mode; they can only be switched to full-screen mode in response to a user action – either a button click or a key press.
Security considerations
End-user security was a key consideration in the implementation of this feature, and developers need to understand the following end-user, security-related restrictions and design content accordingly:
•The ActionScript that initiates full-screen mode can be called only in response to a mouse click or keypress. If it is called in other situations, it will be ignored (in ActionScript 2.0) or throw an exception (in ActionScript 3.0).
http://www.adobe.com/devnet/flashplayer/articles/full_screen_mode.html