A suggestion: If your Panhel will allow, try having only COB rush for a year or two. Have an info table during formal rush, or participate in the first round of parties and then drop out. Then, have your first COB rush immediately after formal rush ends. Get the no-bid list from Panhel and invite those women to informal rush. You may pick up people who went through formal rush and became interested in being in a sorority but didn't like what they saw during formal rush, or who didn't match for some reason, etc.
My sorority was in the same situation - 4 sororities at about 80 sisters each returning for rush, and us with about 10. PNMs wondered what was wrong with us that we were so much smaller.

We did a LOT of COB rush to get our numbers up, and while my chapter is still about half the size of the other chapters at my school, they compete on a relatively even basis now.
One thing we did, was that we asked all freshmen and sophomores in the sorority to invite one unaffiliated freshman/sophomore (roommate, etc) to each COB event. Even if they thought they wouldn't be interested... we've changed a few minds in that regard