I agree with Tom, I have found the more water changes you do the faster a fish (any fish) grows, if you change only a small percentage once per week (say 30 %), growth will be significantly less than if you do two or more 50% water changes per week (or better even more).
I know some breeders that change 90% every day to achieve maximum growth.
Of course it may be that with all the in breeding in the FH court, and messing around with mutant short bodies, it could be just a runt.
But what is your water change routine? its probably the main culprit if its not much, and the FH is not growing.