Remember that COB doesn't always have to be a big event. If a girl can't come to scheduled COB event but still is interested, have her come to the house for dinner, or get a group together to eat lunch with her in the cafeteria. Take her along to a Greek event, or anything that a group of sisters were planning to go to (except perhaps not to the bar
)
For targeting girls that are probably already interested, check with your Panhellenic. Do they keep records of girls who rushed and dropped out? Or lists of girls interested in COB? That gives you a pool of girls who are already at least somewhat interested in joining a sorority. Call them up, invite them to things, make a flyer especially for them and mail it to all of them, etc.
Good luck!