Probably because she didn't want to get caught in the awkward and embarassing situation where you were waving to the person behind her. When you are around her, look at her and let her catch you staring, then smile when she does. After a couple of days of this if she seems to be liking the attention/like you, then go up and talk to her.
because she's either too shy or not interested. OR she just didn't see you...