While a few guys are just bozos, most lose out on hot dates because of ignorance, not incompetence. Essentially, most men just fail to notice when girls are into them.
Though it would be awesome to read women's minds like Mel Gibson in What Women Want, what we're left with is a maze of hand gestures, hair tosses and coded lines. But don't fret. Although flirting is highly individualistic (i.e. you can't hold it against a girl when you guess wrong), there are some signs that are fairly universal. Our partners at AskMen explain 10 signs she's flirting with you.
For more flirting tips, see advice on when you should take office flirtation to the next level.