Chickens won't lay eggs

I have made a small city map for my minecraft, and I have made a farm with sheep, cows, pigs, squids and chickens. I've been waiting forever for a chicken to lay an egg, as it is the currency in the map (using the "CreateShops" filter for mcedit). I don't know why this is, so I'm supposing it's a bug. If this is not a bug then forgive me for reporting this but I am nevertheless confused why this is happening. Thank you,

Yetti2001