Yellowing leaves and new growth?

I have 3 plants out of my 6 that have been yellowing for about a week now... At first I wasnt too concerned as 2 of them are my seed carriers and have lots of seeds but one of my smaller female plants is doing the same thing... So now I guess my question.. Is is nutrient lock/ ph in the soil/ or just starving.. Looks starving to me but I am new to this with a medical card and would definitely like some advice before I screw up a good plant.
They are in week 7 flower they did get stretched for sure I know that and working on it.
I am using LED lighting and heights manufacture stated for tops of plants were burning the tops..
I am using coco/ perlite with jacks classic water soluble plant food.

I had just watered yesterday with 1/2 teaspoon which is half of what manufacturers recommend during flowering.. I feed them a 1/4 teaspoon through veg and it works great. Also checked the water before I poured on plants and was at a 6.8 reading.