This situation did not happen every time. After I created and deleted a large number vms frequently,  odl would not write flow tables to ovs of the new created vm.  Here are the logs. It seemed that the logs were not complete and  some logs were not printed.

2018-08-24 17:27:43,159 | INFO | Pool-1-worker-12 | NeutronPortChangeListener | 353 - org.opendaylight.netvirt.neutronvpn-impl - 0.6.3 | Of-port-interface creation for port 20ecd8bd-6683-4589-b4f1-86b2ebcaac59
2018-08-24 17:27:43,163 | INFO | Pool-1-worker-12 | NeutronvpnManager | 353 - org.opendaylight.netvirt.neutronvpn-impl - 0.6.3 | updateSubnetmapNodeWithPorts : subnetId Uuid [_value=b7ec16d8-2910-4e1b-8cd0-b90baf84ac10], subnetMapId KeyedInstanceIdentifier

{targetType=interface org.opendaylight.yang.gen.v1.urn.opendaylight.netvirt.neutronvpn.rev150602.subnetmaps.Subnetmap, path=[org.opendaylight.yang.gen.v1.urn.opendaylight.netvirt.neutronvpn.rev150602.Subnetmaps, org.opendaylight.yang.gen.v1.urn.opendaylight.netvirt.neutronvpn.rev150602.subnetmaps.Subnetmap[key=SubnetmapKey [_id=Uuid [_value=b7ec16d8-2910-4e1b-8cd0-b90baf84ac10]]]]}

2018-08-24 17:27:43,169 | INFO | Pool-1-worker-12 | NeutronvpnManager | 353 - org.opendaylight.netvirt.neutronvpn-impl - 0.6.3 | Network Id is Uuid [_value=99e42a30-e1d0-431f-8b53-98c57fdd5a8c]
2018-08-24 17:27:43,169 | INFO | eChangeHandler-0 | SubnetmapChangeListener | 362 - org.opendaylight.netvirt.vpnmanager-impl - 0.6.3 | SubnetMapChangeListener:update: update port list for subnet b7ec16d8-2910-4e1b-8cd0-b90baf84ac10
2018-08-24 17:27:43,169 | INFO | Pool-1-worker-12 | NeutronvpnManager | 353 - org.opendaylight.netvirt.neutronvpn-impl - 0.6.3 | Creating vpn interface VpnInterface [_key=VpnInterfaceKey [_name=20ecd8bd-6683-4589-b4f1-86b2ebcaac59], _name=20ecd8bd-6683-4589-b4f1-86b2ebcaac59, _networkId=Uuid [_value=99e42a30-e1d0-431f-8b53-98c57fdd5a8c], _networkType=VXLAN, _segmentationId=101, _vpnInstanceNames=[VpnInstanceNames [_associatedSubnetType=V4AndV6Subnets, _key=VpnInstanceNamesKey [_vpnName=34869b4c-0775-4efb-abd0-d9b7c6b116ed], _vpnName=34869b4c-0775-4efb-abd0-d9b7c6b116ed, augmentation=[]]], _routerInterface=false, augmentation=[Adjacencies [_adjacency=[Adjacency [_adjacencyType=PrimaryAdjacency, _ipAddress=10.10.5.7/32, _key=AdjacencyKey [_ipAddress=10.10.5.7/32], _macAddress=fa:16:3e:07:2d:74, _subnetId=Uuid [_value=b7ec16d8-2910-4e1b-8cd0-b90baf84ac10], augmentation=[]]]]]]
2018-08-24 17:27:43,169 | INFO | eChangeHandler-0 | NatUtil | 351 - org.opendaylight.netvirt.natservice-impl - 0.6.3 | getSubnetGwMac : No resolution was found to GW ip IpAddress [_ipv4Address=Ipv4Address [_value=10.10.5.1]] in subnet b7ec16d8-2910-4e1b-8cd0-b90baf84ac10
2018-08-24 17:27:43,169 | INFO | eChangeHandler-0 | VpnSubnetRouteHandler | 362 - org.opendaylight.netvirt.vpnmanager-impl - 0.6.3 | SUBNETROUTE: onPortAddedToSubnet: Port 20ecd8bd-6683-4589-b4f1-86b2ebcaac59 being added to subnet b7ec16d8-2910-4e1b-8cd0-b90baf84ac10
2018-08-24 17:27:43,171 | INFO | eChangeHandler-0 | VpnSubnetRouteHandler | 362 - org.opendaylight.netvirt.vpnmanager-impl - 0.6.3 | SUBNETROUTE: onPortAddedToSubnet: Port 20ecd8bd-6683-4589-b4f1-86b2ebcaac59 is part of a subnet b7ec16d8-2910-4e1b-8cd0-b90baf84ac10 that is not in VPN, ignoring
2018-08-24 17:27:43,175 | INFO | eChangeHandler-0 | VpnInterfaceManager | 362 - org.opendaylight.netvirt.vpnmanager-impl - 0.6.3 | add: intfName 20ecd8bd-6683-4589-b4f1-86b2ebcaac59 onto vpnName [34869b4c-0775-4efb-abd0-d9b7c6b116ed]
2018-08-24 17:27:43,175 | INFO | eChangeHandler-0 | NatRouterInterfaceListener | 351 - org.opendaylight.netvirt.natservice-impl - 0.6.3 | add : Interface 20ecd8bd-6683-4589-b4f1-86b2ebcaac59 not yet operational to handle router interface add event in router 34869b4c-0775-4efb-abd0-d9b7c6b116ed
2018-08-24 17:27:43,175 | INFO | eChangeHandler-0 | ElanInterfaceManager | 342 - org.opendaylight.netvirt.elanmanager-impl - 0.6.3 | Interface 20ecd8bd-6683-4589-b4f1-86b2ebcaac59 is removed from Interface Oper DS due to port down
2018-08-24 17:27:43,176 | INFO | eChangeHandler-0 | VpnInterfaceManager | 362 - org.opendaylight.netvirt.vpnmanager-impl - 0.6.3 | addVpnInterface: Handling addition of VPN interface 20ecd8bd-6683-4589-b4f1-86b2ebcaac59 on vpn 34869b4c-0775-4efb-abd0-d9b7c6b116ed skipped as interfaceState is not available
2018-08-24 17:27:44,179 | INFO | eChangeHandler-0 | NeutronPortChangeListener | 353 - org.opendaylight.netvirt.neutronvpn-impl - 0.6.3 | Update port 20ecd8bd-6683-4589-b4f1-86b2ebcaac59 from network Uuid [_value=99e42a30-e1d0-431f-8b53-98c57fdd5a8c]
2018-08-24 17:27:44,185 | INFO | Pool-1-worker-12 | NeutronPortChangeListener | 353 - org.opendaylight.netvirt.neutronvpn-impl - 0.6.3 | update: Of-port-interface updation for port 20ecd8bd-6683-4589-b4f1-86b2ebcaac59
2018-08-24 17:27:44,509 | INFO | eChangeHandler-0 | NeutronPortChangeListener | 353 - org.opendaylight.netvirt.neutronvpn-impl - 0.6.3 | Update port 20ecd8bd-6683-4589-b4f1-86b2ebcaac59 from network Uuid [_value=99e42a30-e1d0-431f-8b53-98c57fdd5a8c]
2018-08-24 17:27:44,510 | INFO | Pool-1-worker-12 | NeutronPortChangeListener | 353 - org.opendaylight.netvirt.neutronvpn-impl - 0.6.3 | update: Of-port-interface updation for port 20ecd8bd-6683-4589-b4f1-86b2ebcaac59