Take note of this note from MSDN:
Backing Up the Virtual Machines
Hyper-V uses one of two mechanisms to back up each VM. The default backup mechanism is called the "Saved State" method, where the VM is put into a saved state during the processing of the PrepareForSnapshot event, snapshots are taken of the appropriate volumes, and the VM is returned to the previous state during the processing of the PostSnapshot event.
The other backup mechanism is called the "Child VM Snapshot" method, which uses VSS inside the child VM to participate in the backup.).
Assuming that you are trying to use the "child VM mechanism", have you made sure you meet the critera outlined?
- Backup (volume snapshot) Integration Service is installed and running in the child VM. The service name is "Hyper-V Volume Shadow Copy Requestor".
Windows 2000: Backup Integration Service is not supported.
- The child VM must be in the running state.
- The Snapshot File Location for the VM is set to be the same volume in the host operating system as the VHD files for the VM.
- All volumes in the child VM are basic disks and there are no dynamic disks.
- All disks in the child VM must use a file system that supports snapshots (for example, NTFS).
If all of those critera are met, I would review the the following:
1) Are the VSS providers / writers in a healthy state on both the host and the guest VMs? This article from Veritas will show you how to check. It's essentially these two commands.
vssadmin list writers
vssadmin list providers
2) Are the integration services up to date on the guest VMs? Try to install the integration services, if it is out of date you will be prompted to upgrade.
3) Have you reviewed the Windows backup logs to determine why this is occuring? The default log location is %windir%\logs\windowsserverbackup - additonal logs are outlined in this Technet post.