I agree you can't force a hen to go broody.
However, if you have one of the breeds (Silkie, Cochin, or game) that frequently broods, and thus may have the genetic makeup, and she is mature (especially a hen that has brooded before), you can create an environment that entices a brood....usually this works best in spring, a common season for brooding, but for the really broody types, you have a good chance of throwing them into a brood almost any season.
In poultry, the hormones are set by warmth and clutch pressure (see article below). I've also read that if their calcium intake is too low, they will not go into a brood as brooding hormone release, genetically (if present), typically comes after a high lay period. (My Silkie wasn't entering into her normal brood, and I gave her extra calcium which seemed to help...she shortly went into a brood). I give my broodies some free feed calcite grit to make sure they are getting enough calcium.
I would also keep them on good high protein feed, like 18% protein, as they need overall good health to promote a good brood. The hen needs to build up her body since brooding will take it out of her.
Letting eggs gather in the clutch is actually a good thing as the pressure of the clutch size places pressure on the nerve in the breast bone that causes release of hormones to cause brooding. (That's not old wives tales...that comes from Veterinary science...see article below).
A nice cozy nest with limited light is also very important.
Create a nest that is safe, dry, secluded, warm, dark, and full of nice nesting materials with a clutch of fake eggs (I like Timothy hay best but soft pine shavings work well too). I've even penned my broody girls to limit their access away from that nest. If she just isn't in the right cycle, she'll get cranky. Let her out. If she is hormonally close, you can trip her into a brood.
Good luck.
LofMc
http://proda.worldpoultry.net/PageFiles/28037/001_boerderij-download-WP6781D01.pdf