Try raising light to at least 20-24".. let them grow into it, or reduce if it stretches.. trial and error for that happy zone, but for younger ones they need slightly less than older plants.
you're getting near the point where it should be full bore, though. So, just adjust and keep eye on internodal spacing. Too much light may not show major issues but can slow growth while imperceivable to our eyes that something negative is occurring.
Seems fine, though.