Have you lollipopped and defoliated recently? It looks like each plant has a lot of mass on it. That means it's trying to push nutrients and energy to all of that growth, not just to main branches and stalks.
Also, your plants are still in a pot smaller than a gallon? If so, they might be root bound and in need of up-potting. Which could be another reason why they aren't growing very much.
I hope this helps.