I had a few suffering from slow growth a while back. Not much I tried worked until I "went back to basics" and remember the line, "its all about the roots and the environment.
With that in mind, you should be aiming to give the best environment you can for them.
Temps, 75-82, 78 perfect.
Humidity, 30 to 60 at that age, 40 being perfect.
Light, 3,000 lumens per sq ft (min) of blue spectrum (6,500k)
Air, circulating and refreshed at least every 4 mins.
Those are the basics, get those right and with good genetics and decent nutrients you are giving them the best chance of being strong plants.
JMO mixed with a little fact. W