What breeds are your hens? If they are breeds that are likely to go broody, I have had good luck sitting in the coop playing youtube videos of chicks peeping. It usually takes a week or less to sway one into going broody! It has worked with Speckled Sussex, Australorp and Dorking hens!
I just noticed the date you posted this....try to wait until spring is in full swing. Sitting and hatching eggs in the winter isn't a good idea and the hens aren't usually in the mood - they know best!!
Just to prove the point, out of dozens of Orpingtons I have had over - ahem - many years, I have only had one that went broody, but she was very, very broody! I have had the best luck with Dorkings and Speckled Sussexs - some times more luck than I really wanted....Okay. If you want a broody hen, as has been stated earlier in this thread, I suggest you get yourself a couple of nice Buff Orpingtons. In MY experience, they are the BEST. I started out with 8. I think six of them went broody within a few weeks of beginning to lay. They all hatched chicks. They spent that spring and summer taking care of babies. When fall and winter came, they didn't lay because of short daylight hours. Come spring, they each laid a few eggs and went broody again. At this point I only had four. Year three was a repeat of years one and two. I got tired of constantly trying to break them out of their broodiness and they all went to freezer camp. Bottom line, for me, you can't beat Orps for broodiness. But if you want eggs, try another breed.
Im not sure about the scientific reasons behind it, but we moved them in the moon. We would have the eggs ready in a pile of straw. And that would be in a small area (we used a rabbit cage). We would go into the hen house after dark on the night of the black moon (the new moon). We usually used bantam hens for this. The hen would be put in the cage and the cage would be in a different shed. We usually had very good luck with this.
Maybe it's just an old wives tale but I have heard that feeding them lot's of corn helps in making them be broody. Could be worth a shot.