yeah, as @Texas Kiki (the goddess of backyard chickens) said, take out the scratch. for treats, fruit OR RAISINS RAISONS ARE GREAT work as a treat very well.
the way she is standing looks a bit weird, or is that just the angle of the camera? if she lays big eggs, that could be because she is older. if she goes to bed earlier, than maybe she loses energy faster and is sick? but you said se doesn't seem sick... try picking her up and bringing her over...
don't feel bad, it's not your fault the chick died.
but Maybe next time a chick dies, leave the body with the mama hen for a little while, so she can realize they are dead, so she doesn't think her baby just disappeared