Its the strain bro, I had the same prob with that one. When I switched to my flower nutes all she wanted was more N so I went half and half with veg nutes and foliar fed high N nutes on top of it. Even then it only leveled out the prob, leaves stopped yellowing but the ones that even showed a sign of low N eventually died. But at the same time other parts of the plant show high N, so after about 2 weeks i said hell with it and went strait flower nutes. But going long on veg nutes did seem to help and now the low N seems to be settling. I have a self made foliar spray that will pump some N into them, but with you not using any chems it would burn the shit outta your girls so im not gonna be any help, lol. Only thing I know is if you don't chop off all your lower leaves before flowering then she would have plenty to feed on when the problem arises.