Yeah, that sounds familiar.
Getting brand new people to stick with adult Karate training is hard..don't take it personally when they leave. It can also be intimidating for new people to be in a class with lots of higher ranks, but the conundrum is it's hard to get new people!
Probably one out of every ten inquiries will actually show up, and when you have people call with a list of questions "do you do X", "why don't you Y"..you can expect that whatever you do, you probably won't satisfy those people.
Has anyone out there experienced anything similar to this at your club/school?
Yes, I have had a little dojo on and off for about five years, and one constant is that you will have a core group, and a revolving door of other people, eventually someone that walks through that door may stay, but it may take some time.
So far i'm going with Harlan's suggestion on the flyers also..but i've been most successful simply with word of mouth..emailing people I know..and "tell your friends" type stuff.