Perhaps it is your marketing. Are you getting your message out to the right people, maybe they don't know you are there. Do you have your website on your business cards? Make sure that all of the people that you have done by "word of mouth" know that you have a website so that they can tell everybody else. Maybe e-mail marketing might help. I would also suggest you make an online registration form so that people can do it electronically. I think it would be a lot easier than printing, filling it out, and mailing it. If you need to keep it that way you should have the registration form open in a new window to print from. People might not like highlighting and printing.
Good luck in all that you do