Typically a Windows Server Backup is scheduled to run nightly but will often fail intermittently and then consistently. From a command line one can see the status of the Volume Shadow Copy writers by typing vssadmin list writers . A number of the writers will show:
State:  Waiting for completion
Last error: No error
Stopping and starting the Volume Shadow Copy service does not change the status. A reboot of the server does fix the issue but is not a good solution. The easiest thing to fix the status is to run a “Backup Once” of just the C drive to either a local or remote drive. Once completed the writers all go back to Stable, No Error.