"A watched pot never boils" - so it is with hens, they seem to only go broody when you don't want them to. I don't know how they can tell what you want, but they seem determined to do the opposite. It's easier to break a hen that is broody than to make one go broody.
As mentioned, some breeds are more predisposed to broodiness, but individuals may not have read that fact and never go broody. Once a hen has successfully hatched chicks, I think they are more inclined to try again later, but that's just a theory.
We have 2 broodies with chicks at the farm now. One has 8 chicks over a week old. I don't believe she has lost a single chick, which is amazing with larger broods like that. The other is confined with 3 chicks, but they are younger. There is nothing quite as heartwarming on the farm as a mother hen scratching and clucking as she teaches her babies to survive in the world.