It's mostly genetics. You can see at even a small size when I first got him his head was showing. I keep my water clean, and he eats a mix of Hikari Bio Gold, Gran Sumo Red, and XO Humpy head. Using a mirror for flaring works too.
In the future, try buying FH when they're around 4 inches. If it isn't showing a head by that size, it won't have the monster kok.