When dealing with party chicks it's usually the last person to talk with them that goes home with um. They are a fickle bunch lol.
In my playing days (taken now), I learned that it didn't matter how much time you put in with a girl. Unless she was planning on leaving soon and you were content with a number, it just wouldn't work out. Eventually her friends / family / rest of the night would drag her away. Hitting on girls 30 minutes away from closing time worked like freakin gold. Get them talking and interested while they still don't know enough about you to know that they don't like you. Works 50% of the time, every time.