i too disagree with the statement made by starfishjackedme, i have a 20 gallon tnak with pc and have coralline spreadign liek wildfire!its all abotu watermovemnt a good source of the initial algae, cal/alk levels and as NM said, patience. mY tank's been set up for just over a year now, and i really didnt develop good coralline growth until about 8 months after setup, with teh addition of seachems reef complete and reef carbonate every other day (and i woudl always overdose these two in equal parts!)
good luck
jon