It could have been someone you liked but not enough to invite back to pref or dropped you herself....if she had spectacular grades and involvement.....i'd definately bid her whether or not she came to our last round or not.
I'm not sure if all of this is written down somewhere online. you could always get on npcwomen.org (NPCs website) and snoop around. if you can't find anything....call the closest greek life person to you and ask some questions....i'm sure they'd be happy to help you (well....unless your calling in the middle of recruitment!
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