Hopefully your hens are of a broody breed. Broodiness is uncommon in some strains and not all hens in generally broody strains will set eggs. Some feel that allowing a hen to build a clutch of eggs may encourage her to go broody. Other than that just be patient or get some silky hens.
Silkies are among the best broodies as are cochins but, it doesn't mean they all will and you can't switch it on or off. When their hormones tell them to go broody they do. What breeds do you have?
BTW Silkies don't needs eggs to go broody - when they are broody they will sit on anything, rocks, twigs, or nothing at all. That is the time when you can slip fertile eggs under them, of any breed. They will hatch them and raise as their own.
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It depends on the breeds of chickens you are keeping. Some breeds are very broody while others never go broody. Frequently broody breeds include orpingtons, silkies, cochins, sussex, d'uccles and brahmas. Some of the best ways to encourage broodiness includes: placing fake eggs in the nests, making the nests dark, putting lots of comfy bedding in the nests and again, making sure you have the appropriate breeds.
Broodies unfortunately can't planned, they just happen. I would also suggest silkies if you want a bird it's almost guaranteed to go broody whether you want them to or not.