Hens do not want to hatch each egg. Most will lay the egg, and never look back. Then go back and do it again the next day.
A setting hen, or a broody hen, is a gift or a curse from the Gods. You cannot induce it, and it can be dang hard to break it. When a hen goes broody, she changes due to the hormones. She will sit in a nest day and night, with only occasional forays out into the real world. She will get an almost trance like look to her eye that I have seen refered to as 'stink eye' When you approach the nest, she will puff up to about twice her original size and growl, and may peck hard at you or another hen.
Sometimes, they kind of play at being broody for a couple of days, before they really settle down to it. But once they do, you will recognize that they have something else on their mind. If you put fertile eggs under her, 21 days later, you should get chicks.
A broody hen no longer lays an egg each day. She stops laying eggs, which is why people sometimes don't want them to go broody. She will not lay an egg for about 6 weeks I think.
I hoped all spring for a broody hen, got two the first of July. They are sharing the duties of 12 fuzzy chicks, and doing a great job. So if you get one, it is fun to hatch out eggs, but you have to wait until they are ready.
Mrs.K