Why does she ignore me?

Obviously she did pick up on you ignoring her. She's probably changing the way she acts because she wants to play hard-to-get. If you have any interest in this woman you should show her. Absolutely do not wait for her to come up to you because she is clearly shy. She is probably waiting for you to show some interest in her and make the first move. Almost all girls are the same when it comes to the guy making the first move. Plus they might not like making the first move because they are afraid of rejection and that's probably what's going on with her. Don't be afraid to step up to her. When you ignore her like you did at first and then started to wave and say hey she probably thought only when it is convenient for you and that's probably why she changed. If you have any interest in her take her out and show her or something. Do not just sit there and wait for her to do something. Be an effin man! Jk. Lol. Be a gentleman like back in the olden days and do whats right.