Do you have deferred rush or can first semester freshmen pledge?
As far as the joint events not generating interest...make sure that what you are advertising is what you actually have to sell. "No matter the letter, we're all Greek together" is a pretty sentiment, but if the day to day reality is trash talking, dirty rushing and a general lack of unity, rushees will see that and be completely turned off. End of lecture.
Also make sure the events aren't being held at the same time as something important (like a Steelers, Eagles or Penn State game
).
I'm in Pittsburgh as you can see - PM me your school if you want and that might help.