Sativas appear to be stressed - can't figure out what's up [images inside]

Hey peeps,

I'm now entering my 3rd week of flowering and the sativas of the bunch appear to be struggling but I can't figure out what the problem is.

Image gallery here :

http://imgur.com/a/g1nEo

My environment seems fine. Plenty of fresh air incoming, 2 fans in here circulating the air. Temps are 23-24c, humidity 50-60.

Initially I thought it was a PH problem and I gave one in particular a good flush and it definitely perked up but the downward leaf curl and shrinkage persists. The soil PH is around 6 (can't find my log book at the moment to find the specific) but it was at 5.4 a few weeks back.

There is no obvious leaf discoloration on these but they're a darker shade of green compared to the indicas I've got in there.

Any help would be appreciated.

Could be Too much N!! because that they are darker brah
But in my opinion looking the pictures they are not so dark green, It's hard man I had the same problem with one sativa I could not find the problem I just kept everything running well they got better after a week but in my case I have a hydro system

i,m growing a sativa aswell m8 and it is stressed to fuck also,same curling leaves like you i thought ph so i looked at that and found nowt wrong , is it possible that they just want more water ? cause mine is a greedy mare !!!!! i had to change the water to twice a day ,1 in the morning after the light goes out and 1 just as the light comes on i hope yer babies get better soon

Nitrogen toxicity. "the claw" like you have going on with that sativa usually means one of 3 things, too much N, a nute lockout causing the nute levels to rise in the soil, messing up the ph and causing the upper leaves to "claw" like an eagles talon or in the last case...a rootbound plant starving for room. Sativa's in my experience are more vulnerable to high N because of their long slender leaves. if you think about it they should need less N in flower because of this reason. Indica's having much broader leaves behave a little better if your N levels are too high in flower. That all being said your ph is too low for soil, try to get it up a bit like to 6.7. Also what size pot is the sativa in? If its in too small a pot it could be rootbound. This is just my opinion, from my experience. She should still do fine although the growth will be stunted and buds a little less dense and more harsh because of the high N levels. Good luck!