It's not often that Glance needs troubleshooting. There are two common ways that you will have things fail related to Glance:
If Glance cannot access the filesystem that it will be writing to when it is saving an image into the registry
If Nova cannot get an image that has been assigned to launch an instance with
In the event that you are not able to save an image to the registry, you will just have to read the logs in /var/log/glance/*. Depending on the backing store that you have chosen, or that has been configured for you, there will be different errors. In most cases, when you resolve these errors, you will have a working Glance service.
When a new instance is launched on a compute node, one of the things that the Nova compute service does is to check whether it has a cached copy of the image that the instance is being launched from on the compute node. If it does, it will use the local cache; if it does not, it will connect to Glance and download...