But children who do not have low levels of that hormone can be taught/night trained.
OP, perhaps have your child assessed by a medical professional to determine their levels of this hormone?
ETA, I wonder how many parents are fruitlessly trying to "train" their children when in fact it's it could down to hormones in some instances. I wonder why it's not more widely heard of? Is it rare? Hmm, just thinking aloud now.