The main thing you should be worried about is the fact that you have been flowering 20 days, yet the plants are definitely not flowering. The whole plant is dark green as well. Also, looking at the pictures again, there are only 2 leaves that are twisted, the others are curled under. Your answer is too much Nitrogen. Not sure how that would have happened with half strength nutes, especially since you shouldn't have been using any Grow Big in the first 2 weeks of flowering, unless something got dumped accidentally or such. Or, since you are using OF soil AND feeding nutes, the plants very well could have been taking in way too much N and it's really showing its ugly face now.
some may disagree with me on this cure, but you should flush them. then feed (PH'd water of course) and a little under 1/2 strength nutes, and slowly (like spread it out through weeks) work your way up in nutrient strength.