Just taking a stab at this....
Drooping can be lots of things, the most likely is over/under watering and under feeding.
Underfeeding causes droop and lime green growth. Doesn't seem to be the case from the pics.
Underwatering seems hard to do in a hydro system. If your doing a flood/drain you could be waiting too long between floods, but you have probably thought of this.
The room temps sound fine. What is the water temp? Is it oxygenated enough? Have the room temps grown since you started to bud? If the water temps have risen as well, it may not be holding enough oxygen. Colder water holds more oxygen and discourages nasty stuff. Water without oxygenation would mimic over watering and cause droop.
Are the roots white and crisp looking, or brown and mushy? Root rot can advance into a plants system and cause drooping, although this takes a while. Algae slime can coat the roots and interfere. I personally would not be comfortable depending on that red plastic lid to block the light. I would cover it with dark cloth, tin foil, trashbags or something.
I would rule out these first.