When watering it is best to thoroughly water then allow to go tinder dry, which is not scratch the surface dry, that means very light pots. That way the plants spend little time in soggy soil which they hate, and moisture is evenly distributed throughout your pots allowing roots to spread evenly through the root area. From the looks of the leaves of most of the lower part of the plant are yet to reach and extend over the sides of the pot, which is a good indicator that the roots have not formed enough yet and/or filled out the pot.
Using the limited method of watering that you are using often results in roots heading straight to the bottom of the pot to find moisture, then clawing their way up the sides, this can slow plant growth while it waits for the roots to amass enough in order to support fast vegetative growth. In thoroughly watering your plants you allow the roots to fill out the entire medium, as well as washing any excess salts that may build up, out of the pot with the run off that comes out the bottom. The next time you water, try watering from the outside rather than around the center.
Secondly the humidity is way too low for vegetative growth and will be the cause of the curled leaves. You may need to mist your plants from time to time to raise humidity, or fix the source of the low humidity. 50-70% is about the range you want your vegging plants to be in right up till when you switch them to budding.
