Chiropractic Marketing Agencies

As seen in the title, I've been looking into hiring a marketing agency to speed up patient flow in my office. This is my first time hiring one of these agencies, so I don't have any experience with it. My budget is pretty flexible right now, but like anybody else, I don't want to waste time and money on a marketing agency for little to no results. What agencies have been either really good or really bad for your practice? Thanks!