Hi Vicki.
Your hens are not sitting on their eggs to hatch them because they are not broody at the moment. They may or may not ever be broody for you. Some hens are, and some aren't. It's a hormonal "condition", not just an opportunity because eggs are in the nest.
Tell us a bit more about your hens. Some breeds are more likely to brood than others. Also, in my experience, birds acquired from a hatchery are less likely to brood. Not that it's impossible, but less likely.