If its all the way in, you can't push on it anymore without using "I'm gonna break it" level force, then chances are, its all the way in.
Some things you might want to check are, if any of the pins are bent on the VGA connector (the one that plugs into the video card). i've seen these get bent, although its rare, and that obviously won't work. Also, if you have another monitor you can test with,that'll determine if the problem is in your monitor or in your mac ...