My husband made our chickens a roosting rack we saw pictured in a book. It is like a slanted rack, with slats across like ladder slats and chicken wire between. We have a pull out tray below it to collect the poo.
They have always loved sleeping on this from almost the day we put them out there as chicks. They never sleep anywhere else. It is roomy enough for the amount of birds we have so we are probably in a similar situation as the people who posted above me, we have plenty of roosting space.
Maybe you just don't have enough space. My chickens cuddle up together at night and act all loving, it is really cute. They never fight. When you look at our space when they are sleeping, you could count up at least four or five empty spaces on the rack the way they bunch together.
Why don't you try doubling the amount of roosting area you provide and see if they don't all settle down.