I just think, from watching a good number of girls/women over the whopping 27 years of my life (I'm so OLD!!), that the majority of women who say they don't/can't flirt are usually saying it AS a way to flirt. Playing "innocent" or "coy". It's usually said to a guy in such a way as to make that guy start flirting with her, to "teach" her, I guess. I don't know.
But! Like I said, there's a few ladies here who have said they just really don't know how to flirt, and I believe it...from a couple of those few (you're one of them, Rachel- I can't see you trying to be coy or cutesy or pretend to be anything you're not).
Oh! And yeah, I do feel that women are, in general, born with a natural ability to flirt. I think, though, it takes a certain amount of attraction to bring it out in some women.
THAT ^^^ is a little bit of Cristen Theory for you.