Check the volume status by using openstack volume show command about the volume (it should have pvc- prefix) and see whether backend openstack need update, contact with your openstack administrator for help on the error reported.
openstack volume message list --os-volume-api-version 3.3 might also show some information about a failed resize, e.g. extend volume:Compute service failed to extend volume.
Chances are, the underlying OpenStack is not configured properly. This could be tested by manually creating a VM and Volume, attaching and resizing the Volume. If you see the same behaviour, no change of the Volume in the VM, try to get in contact with the Admin-Team of the OpenStack you are using.
Chances are, Cinder is not allowed to send the volume-extended event to Nova API.
The error likely can be spotted as a 403 in the logs.
In that case. The section nova in cinder.conf
must be configured properly, and with a user with sufficient privileges.