This problem is very vague and can be caused by numerous reasons. Here are the steps to debug(hopefully you will find something):-

Note:- i am assuming you networking topology to be of VLAN type so in case its different then let me know

1. Openstack creates network namespace for every network. Go to the network node and see if there are network namespaces present( You can identify your name space by you network id, Do a neutron net-list and note the id of your network)

2. If namespaces are created then again on the network node see if dhcp server is running (ps -aef |grep dnsmasq).

3. if dhcp server is running then in when you will do the grep in step 2 you there in the process information given by the ps command you will get a host file to which dnsmasq process is attached)

4. open that file, you will find the IP of the instance which you created along with the Mac address). Cross check that.

5. If you are not this step then I assume that till step 4 everything is working fine and in order. In that case lets move to OVS bridges. There will be a data bridge normally br-data-eth or something on the network as well as compute node, check if they are present( Note the name can change but it should be what you gave in the bridge_mappings in the ml2_conf.ini file)

6. if bridges and created on neutron and compute node and are also in the configuration file then it means that under the bridge the ehternet adapter is the culprit.

7. On neutron node ovs-vsctl show will display what ports bridge contains. Check the interface which is inside the data port is connected to the compute node.

8. perform step 7 on compute node as well and ensure that the data bridge on the compute node has an Ethernet interface added as a port and is connected to the neutron node.

9. If all the above things are configured properly then it means that the flows of OVS are not created ovs-ofctl dump flows <bridge-name> will provide you with the flows.

I think it is enough debugging and you will find the culprit :) , if not let me know what issue you are still facing