I have two. The big one does grow but slowly. The small one I bought about 7 months ago. When I got it, it was the size of a nickel, today it's about 2.5" diameter. I never feed it, though it catches plenty on its own.
All corals are the same. In some tanks they seem to grow and in others they don't. There are a lot of factors besides food. Lighting, waterflow, water conditions. So try changing something and see if it helps. But remember this is a hobby that takes patience. It takes months to see if a change makes a real difference.