I don't think it's you dai.....i think it may be your seeds and/or something about coco that I just don't know about. But I wouldn't put it past those buggers to just grow funky to make life hard......a good half of mine have been useless devils.

edit: I don't think it's your lighting but how far away from the mh are they? They don't look burned.....it's just looks like a ph thing. They're curly even for my freaky little devils

PS....you've recently calibrated your ph meter of course.......just typing out loud......