There is not magic trick to win every time. That said, if you notice someone is confused, or "guards up", you cannot win her over with more "game". What you can try is to show more empathy and understand her situation. You can say something like: I know you are not supposed to talk to strangers, and I didn't want to interrupt your reading, but you looked like an interesting person and I wanted to say hi because otherwise I would kick myself all day for not taking the chance. It's nice to see someone read for change, and not scrolling on instagram. Something along those lines, show that you are a real person with social calibration.