They will eat some grass if that is all that is available.
Many backyard keepers believe that they eat grass because they once had grassy lawns and now have bare areas. The chickens eat little of that grass. Most of the loss comes from scratching soil looking for bugs and seeds and digging up...
A lot of people talk about chickens eating grass. Grazers eat grass and can get most of their sustenance from that. Grazers include horses, cattle, bison, sheep, rabbits and grasshoppers.
Other herbivores are browsers(leaf eaters), frugivores(fruit eaters), granivores(seed eaters)...
I don't want conflict. I want to help.
Before you said they wouldn't eat it, you didn't say that you fed any chicken feed so I figured you couldn't obtain any.
Birds that start healthy and have complete nutrition and big ventilation don't get sick unless there is a nearby flock with an avian...
Let me clarify. I understand that you like the foods you are feeding but they aren't necessarily providing the nutrients they need.
When you give seeds, potato and grass, do you know what the ratio of calcium to phosphorus is? Do you know if they are getting selenium? Are they getting enough...
Is this bird actively laying eggs?
Egg is ok, potato and yogurt are not really chicken food.
Trust me on this and provide the nutrition chickens are known to need and see if the condition goes away.
Are you unable to obtain a chicken starter, grower, all flock or layer feed?
Well maybe there's your sign.
Is there no chicken feed available where you live in the world?
Chickens are omnivores, not specifically seed eaters and grass is usually avoided and has little nutrition for non-grazers. If they consume greenery, they prefer tender forbs like clover, alfalfa...