Unfortunately, what you have found and the others have said are true...based on experiences with different alumnae chapters of my sorority.
It is very very frustrating to get excited about an event, either just the alums or with the undergrads, send out 100's of invitations and only getting a handful of people to attend..ususally the officers and their pledge sisters.
I agree that it is probably common practice that alums ignore their own chapter or one nearby or where their daughter/son goes to school, until kiddie wants to go through rush and then it's BY GOD, I'M AN ALUM AND SHE/HE IS A LEGACY!
Looking at another sorority in town, they have 2-3 different alumnae groups because they are taught as undergrads about how critical and expected that YOU will attend Alumnae meetings.
If undergrads don't see an active alumnae base while in school, it may be hard to create a vibrant, active local alumnae chapter.
Unfortunately, that leads to many alumnae (and sometimes undergrad) chapters going inactive.
It can be only a couple of hours or more a month..not bad for a "lifetime committment".