Under Cupped Leaves, Need Help!

My plants have a few under cupping leaves, some looking as bad as an upside down V; the leaves that are affected also point to the floor and curl slightly inwards. All symptoms only appear on branches on lower to mid portions of the plant, no discoloration yet.

Additional info:
Plants are at least 3 months, possibly younger, generally healthy otherwise.
I water them with Cullgan water just when a couple or so inches of soil is dry, usually 7 to 9 days apart, with properly phd water suited for soiless growing.
Light source is 400watt hps, a foot or so away.
Temperature stays around 21-23 Celsius with 40ish humidity for flowering.
Plants are 2 feet tall and almost a week into flowering.

Hopefully this info helps you, ty

sounds like its to hot for your strain or they are thirsty....Do you mist them when they are cool or when they are warmed up by the hps lighting???

Do the edges of the leaves curl inword. Very hard to diagnose w/ out pics. Does sound like maby a heating prob.

I don't mist my plants at all, might as well try it.
The leaves that look ill only curl slightly inwards, they kind of look claw like or an upside down canoe.
If it helps you guys im growing Northern Lights #5.

Its a 50/50 chance its nothing, but I would guess that they are stressed over something. The leaves tend to do that at night. But come back to attention when the sun comes up.

when you mist them make sure the plants are cool like right when the lights go on or else they cant ventilate "breathe"