I personally have found when coralline isn't growing its because Alkalinity levels are low.
How you get them higher is up to you (Purple up, pickling lime / Kalkwasser, Reef Builder, or plain ole Arm and Hammer Baking Soda). Calcium is also important, but most newer systems will have enough calcium from regualar water changes.
As mentioned above you will also have to seed it, either from the LR you buy, snails shells, etc.
Good luck and keep us posted.
Brandon