I wish i could give you some advice on that, but from what i can see your doing everything right. Your plants leaves look very healthy! They may just be curling because of their weight, or genetics possibly. If you got more seeds, and some time, you could just start flowering now and see how it goes. Man, im sorry this grow isn't turning out so great. don't give up, your hard work will pay off in the end! hydro can be tricky, but its totally worth it.