[Gusteau]

Are your members friends with sorority women?

This, in my mind, is the biggest thing. If your members aren't actually friends with the women in many different sororities there's no one to pull for you when they're voting on who to mix with. You don't want them to say, "Does anyone actually know an XYZ?" ... "No. Moving on." Getting to know them is a big action point to turn things around.

Other things you could do: send a nice (handwritten!) card to congratulate them on initiating their new members, send a bouquet of their flowers on their Founders Day, attend all of their philanthropy events, have your pledge classes plan a social/philanthropy/fundraiser/etiquette dinner/etc. together...the list goes on.

Another thing you might want to try is asking a sorority to pair up with you on a philanthropy event or fundraiser - it won't count against how many social events they have, but for your chapter it can be a social opportunity.