what everyone else said, but they forgot the main recipe: patience. it will probably take a few months, so just give it some time. but, here is one additional thing that you can do to help speed things up. turn off your filter(s) and just leave the power heads running. take a razor and scrape off some of the coralline that you do see. let the power heads circulate it for about an hour or two until the little scrapings land in different places in your tank. then turn your filters back on and wait for it to grow.