Moved to FAQ!
#1 Genetics- Does your FH come from a line w/ good Kok potential?
If yes,...
#2 Temperature- 86 to 90 deg. F High temp will increase metabolism.
#3 Power feed- Feed until FH will eat no more. (Clean away all uneaten food and maintain clean water)
#4 pH- Ensure that your tank water is at a good pH level
#5 Aggressiveness- The more aggressive your FH, the bigger its Kok will grow. Try using another FH that it is aggressive towards, seperated by a divider. Or if your FH is in a large tank, use small fast swimming fish to allow the FH to chase after. ...That'll make it feel dominant and the head will

.
There are many many factors. ...It is best to read up on many of the past posts before asking. It may be fast and easy to just ask, but much of this has been answered time after time.
Good Luck!
Dennis
P.S. Post some before and after pics.