From my experience with a similar non-profit group (thirty years with
the American Society of Media Photographers) as both member and chief
officer, I can only concur with this observation. You need to approach
people individually and then STAY involved. Mentor them. Take them for
a flight. Share your own soaring experiences with them. (I have an
email list of friends to whom I periodically post flight accounts.)

Broadcast recruiting seldom works -- personal contact is highly
effective. It takes a little energy, but enthusiasm is highly
infectuous.