does she have eggs under her? or do the other hens keep laying new eggs and she has those new ones under her.
you might just have to take her up if she isnt sitting on anything. altho i had a hen who i thought was overdue by 4 or 5 days...and then she hatched! so you might want to give her a couple more days.
there are a couple ways to "break" a broody... i had to do this - see here:
http://adventuresinthegoodland.blogspot.com/2010/08/good-bad-and-broody.html
especially in the comments where i explained the dunking.
if you take her up - or her eggs just dont hatch (do you have a rooster?) - she might be nuts for a couple days to a week. she may do that funny fluff up strut and clucking for a while. just let her be.
hens handle their broodiness differently. for instance i have one screw-around-gal who is always going here or going there.. and hatches no problem. then i have a gal who is locked down. she doesnt eat or move. put a small (cat food can) dish of water and food near her so she doesnt have to get up.
if she has to poop she'll either hold it the entire time, get up while you arent around, or poop on the nest (which is a problem)
if you arent home during the day she might be up and around while you arent looking. sometimes they will sneak off, do their thing and get right back on the nest.
it takes 21 days to hatch a clutch - so give her the full 3 weeks (plus 2 or 3 days) to see what happens.
good luck!
