Recently we had a case where we tried to restore a VM and it failed. Although all backups finished successfully. We also noticed that single file recovery from that VM was not available. After taking another look at the backup jobs we noticed that the affected VM had only backed up 5 files instead of thousands of files which is normal when “Enable file recovery from VM backup” is enabled.
After some investigation together with Veritas we discovered that Changed Block Tracking (CBT) file was corrupted. We deleted the cbt files from VM directory when VM was powered off. After VM was powered on again new cbt files were created. After that everything started to work correctly.
VMWare KB article about enabling/disabling Changed Block Tracking (CBT) – https://kb.vmware.com/s/article/1031873