She can see that you're there to either pick her up or sell her something, and her anxieties about the awkwardness that will ensue override her normal politeness in talking to you. You need to override that anxiety by being more interesting and persistent in your approach, possibly less creepy/weird. On the other hand, some girls will legitimately completely ignore you because they're having a bad day or whatever reason. So don't let it get you down.
From what you are describing, it seems like you are not being commanding enough in your approach to inspire people to talk to you. Watch this, it illustrates this point wonderfully:
http://www.youtube.com/watch?v=Xzrc8fZISDo