when i was feeding my chickens only hen scratch which is barley, wheat, split pea,
flax oil. I also gave them this chicken food remix stuff which is basically what they were putting in the pellets. It contained the salt, calcium, phosphorus, b vitamins, copper, etc,etc. They got this along with being fed oyster shells. We pasture raise our chickens so they received new grass everyday.
Are you only feeding them the oats/lentils? Have your tried giving them sea90 or kelp meal?
What else do these chickens have access to eat?
I have noticed people where i live who give their chickens minimal food variety. they offer them also a lot of land to scrounge the rest of their requirements from. be it plants/animals,bugs,poop, etc.