Difficulty in connecting external projector to Macbook Air

I have an external projector I'm connecting to my Macbook Air (OSX 10.7.2) via mini display port to VGA adapter. With projector on and ad adapter plugged into the macbook air, I see the screen flash blue, but can't force the external dispaly to be recognized. Under system preferences, displays, I am hitting detect displays, and still nothing. I have gotten the plug-n-play capability to work before using the adapter, but even after system reboot, I can't get it to work. Any suggestions?