In simple cases, you've created a network but dhcp and network ranges aren't showing up in the web ui. For that, re-start the dhcp agent:
sudo /etc/init.d/neutron-dhcp-agent
When dhcp doesn't seem to be working, or when the uuids in the logs and device names in the logs on the hypervisors don't seem to match existing names of tunnels and bridges, do this:
stop the dhcp agent, stop any dnsmasq processes, remove the ports
(ovs-vsctl del-port br-int tap83ba5659-b4 and also in neutron),