Plant does not drink nutes500ppm runoff

roots are not selective eates... anything that fits is absorbed, in the simplest terms. runoff is not what was in teh soil nor what is after watering. it's some weighted average of what you poured plus what's already in the pot (evaporation increases concentration of what is left) since last time etc etc... the point is it will alwys have an EC somewhere near what you add if doing things right... higher ec is more likley. proper irrigation for coco is simple... water entirety until you get 10% runoff. rinse and repeat. using weight will be th emost consistent way to trigger an irrigation. if you irrigate at a consistent weight, it will requie a consistent amount of water to achieve 10% runoff. always feed 'full' strength in a soilless/hydro context with mature plants. These can definitely handle a 1.3-1.5EC properly balanced feed. feed that every time.. observe and adjust mix of fertilizers from there... Find that well-balanced happy-zone and the worst-case scenario is a 1in 20 picky eater with mild symptoms that are easy to diagnose with simple fix. If you've been doing that and it's getting good light then it could just be a dwarf or runty plant of some sort. Looks like 2 of them are falling behind the others. They are on edges, so could try moving them to the center, but imo, i'd keep the one in the coner and move the other to a corner too... better to maximize light on your best plants.

The two smallest plants are also two of the plants furthest from the centre of your light. I think this issue is a lack of direct light, not nutrients...........one 300w light for 12 plants may be asking a bit much. Try placing the smalller plants in the middle for a week or two and see if the growth catches up. Rotating your plants may be the simplest solution at this stage.