Just be patient. As long as you maintain your calcium where it should be (400-450 ppm) your coralline will eventually take off. I've also heard that it can be sensitive to pH, and I noticed this in my tank. When I was battling my low pH problems, it almost completely stopped growing, and now that I have corrected it and my pH stays in the correct range, the coralline has taken off again.
Some people say that adding strontium will help it grow faster. I tried it for a couple of months and never really noticed a difference. I stopped adding it, because I don't like adding things to my tank that I can't test for.
I've also heard of people taking a rock with coralline on it and scrubbing it while holding it in front of a powerhead. Supposedly, this knocks the spores loose and spreads them around the tank, so they can land on other rocks and start spreading. Again, I've never tried it, but heard that it works.