0kb for VMs on Hyper-V 2012R2 Windows Server Backup

I've a Windows Standard 2012R2 Hyper-V Host with 5 VM's (assorted server 2008R2 up) which after various changes and cleanups etc now does a full VSS backup successfully. The server is a 4x500GB RAID 10 and backups up to a 1TB USB and reports success. When i look at the details they all complete successfully however on the 5 VM's it shows a data transfer of 0kb? Is this becasue it snapshots and includes the snapshot with the full backup? Am i missing something and its failing?