I find with my own chapter which is the same size as yours and is on a medium size campus is the fact you MUST advertise like mad, i'm talking blanket the campus, you also have to exploit your resourses, other non-greek friends get them to go and bring others, also if possible announce the event to people in your classes. Sounds dumb and strenious but it will have the results. At least then people will know who you are on campus, can't gurantee they will like you but they will know who you are at least
That's my two cents as well
Ed