[Marvin]

Can anyone help as to why one of my chickens has stopped laying?  She is a two year old crossbred bird who normally lays very large, brown eggs every other day.  I haven't had an egg from her for over 3 weeks.  She is eating, drinking and her comb is firm and colourful, she isn't very happy she sits along a lot and her tail is down.  She, along with the other birds is wormed regularly and they haven't any mites.  She doesn't feel to be egg bound either.