Lana,
We had a huge problem with this while I was actually on a study abroad my senior year. I came back and NONE of our alums wanted to help because the president and chapter had been so rude to them that they wanted nothing more to do with our chapter. Did that ever mess us up, and having no alum support makes many events horribly difficult.
I know know how your org is structured, for instance is there a membership education chair? Have an education session for the house where you have some alums(if you can get some) speak about their collegiate days, both recent and older alums. Start the education process with your chapter and have it the same type of education with your new members.
My chapter now knows how invaluable alum support is, and thankfully have changed the way they do things,
much to my delight.