Get a copy of
I Heart Recruitment by Coffey and Gendron. It'll make you rewrite one of your sorority's "favorite sayings".
Like 33girl said, it is great that you've realized you need to stop relying (solely) on PNMs coming to you. Go to them. A COB-only recruitment system will only hurt your chapter if you rely on formal recruitment for publicity, but that doesn't sound like it's the case, since you've been takin 2 NM classes each semester.
Get your members to befriend more women on campus. Hang out like normal friends do, individually at first, then gradually introducing them to a few of your sisters, but still within the "friend" schema, not the "PNM" schema.
Your members are probably involved in other activities on campus. Leaders of other campus groups are a great place to look for quality members. When you have philanthropy events, invite members of other campus groups to come.
Is retention also a problem?
A minor note...maybe it's just really popular where you go to school, but I'd personally be VERY turned off by a sorority that had a hookah party for recruitment. I'd cross that chapter off my list for sure UNLESS I was already friends with someone in the chapter.