That's a good feed, to include kelp. If it's pelletized though, or in crumble, it'll be cooked, and needless to say cooked feed is less healthy than raw feed. I personally wouldn't use cooked kelp. A fair bit of its minerals would still be present in good form but other components would be altered and damaged. Since it's already in their feed though I wouldn't supplement them with raw kelp because they could receive an overdose of something or several somethings which can be as fatal as an underdose, except overdose is usually quicker acting. Same symptoms though in most cases.
Some strains of chooks don't produce well on certain feeds, i.e. some don't cope with soy and legumes well. It might help to do a complete check of all the unusual ingredients in your feed, as in check around on the internet whether anyone else has had birds fail to lay with such additives in their diet. Just a random idea, might be irrelevant. Another theory is that the kelp in your feed is too low to achieve the standard dosage per bird, which is a pinch per adult bird, per day. If any of the non-laying females are on your to-cull list, maybe a cull and autopsy would reveal the cause.
I found that when feeding kelp for a few generations all the bird's crests were intense blood-red, and the males had deep cherry-red crests; not a pink crest among them. Even the week old cockerels would get cherry red crests. The moulting stopped exposing bare patches and they had strong enough feathers to cope with the gender ratio often peaking at 50:50. I experimented with the kelp and stopped feeding it to them, and rapidly they lost the intense crest, wattle and face coloration and began moulting badly (normally) again; went back to feeding kelp, and they recovered, so I know it's the kelp in that case. But even if yours are getting enough kelp something else can be holding them back.
If these non layers were my poultry, I'd separate a test hen out of the non-layers and feed her plain grains, some raw stuff like grass and garlic, non pellet/crumble protein, and a pinch of granulated kelp per day mixed in with her feed. Soon enough you'd see if it was the form of the kelp being administered that's at fault, or something else in the diet. But it does sound more severe than a mere deficiency, since even hens on a kelp-less diet ought to lay. There are some diseases that can cause cessation of laying. It's possible also that you've bought hens from a breeder who carries a line of birds that tend towards sterility or intersexed birds, so they're not full females nor full males. There's a lot of things that could be behind this. I would check their water sources, the containers they're fed and watered in, and the soil and buildings they live in, as well as all plants and insects they have access to. Excess estrogen can cause infertility in females too, so if a plastic container is leaching too much, that could be it. Various chemicals could be causing this, too. Certain fungi in the environment can also cause laying to cease. As always some individuals are more susceptible than others.
Best wishes and I hope you find what the issue is.
