mine grew from 3 to 6 inches in 4 months before my oscar killed him while i was on vacation... upping water changes always helps increase growth rate with fish, but green terrors particularly will get a MUCH increased growth rate IME
you need to do water changes more often, gts release a hormone that keeps them smaller if there are too many fish in the water. i bet if you upped your water changes it would start growing. also what size tank you got it in?
my green terror grew slow at first, but then when i introduced my JD to the tank, he went fom about 4" to 8-9" in like 3 months, he was awesome, but his reign of kingship ended when my oscar decided to get massive -_- i want another terror