If she's been broody for about 2 weeks, she will likely accept chicks. If she hasn't been broody long enough, it's a real gamble. If you want to break her, the cage needs to have a wire floor, and it needs to be up off the ground.
Could be too many extras. Treats should not be more than 10% of the daily diet. Too much corn or scraps can cut into the protein intake. Without enough protein, they can't produce eggs.
If one is sitting in the nest box 24/7, she's broody. Could the other be molting? Has there been any possible stress? Could there be a hidden nest somewhere? Have you been feeding a lot of treats? Sudden change in temperature?