What type of feed are you buying? How old is it? Is it rancid or spoiled? Have you tried other brands or fresher feed? How long did you leave the feed for them to eat?
The age and quality make a difference.
Chickens will always choose the tastier scratch, which is high in fat, carbs and calories and low in vitamins, minerals, protein. It's like I said, junk food for chickens. it should be less than a 10% of their diet. Diets high in carbs and fat cause fat to build up on all of us, but on birds, the fat builds up internally, around the heart, liver and organs, eventually causing them to fail.
Poultry nutrition is one of the highest researched areas of the studies on the planet (every country researchs it heavily), if they could thrive off scratch, we'd feed them that. But they can't in most environments. Unless you having roving flocks of feral chickens in your area, your modern breeds will not get enough nutrients to thrive as they should.
We can show how to find these studies so you can make your own choices I but get the feeling you're not really interested in it anyway.