windows instance can't get a dhcp ip and this break my network environment

I have two nodes, a control node, adn the other running nova-compute services. Network environment is flatdhcp.
  Launch a instance of ubuntu, every success. But if I launch a windows instance, windows does not get a dhcp address, and then launch an instance of ubuntu can not get dhcp address.
My windows made ​​as follows: http://docs.openstack.org/cactus/openstack-compute/admin/content/creating-a-windows-image.html
I could not see from the log which clearly wrong, and I can not see an instance of launch windows firewall rules around what is the difference.

Hope for your help