It's hard to say, because it could be different things that are the opposite of each other.
Chances are, as young as they are, you're being overzealous and feeding them too much.
What are you feeding them, and do you know your EC or PPM? If you don't know, can you tell us exactly what you mix and what kind of water you use?
It's also possible that you have fed them correctly, but with pH fluctuating in a way that caused them to lock out something and "go hungry" despite being fed.
Or they could have a deficiency in Ca or Mg or both. Or they could be getting burned by that light that's big enough for flowering 4 foot plants and is possibly way too much for these little babies. Babies don't need that much light and it can work against them. In a natural grow they do way better with a nurse plant giving them shade at this stage.
I'm envious of your temperature. Here in Arizona we dream of a 70 degree room with a HID light.