I think someone may have responded to you re: this in another thread but I'll reiterate.
There are three possibilities:
1) The school could have a rule, either the administration or the panhellenic, that grad students are not eligible for recruitment.
2) The sorority iteself, either nationally or locally, may not accept grad students.
3) Not likely, but its possible the sorority was using your status as a reason to let you down easy.
In terms of what to do--if you really want to "die on this hill" I would start with the school and make sure they do not have the rule. Ask a recuitment counselor or an administrator to assist you in determining local and national rules.
In terms of AI--read the READ THIS FIRST thread at the top of the AI forum. Discretion is your friend.
Best Wishes!