AV does not detect malware, but"Virus Total"shows: Comodo: "Unclassified malware

I have just scanned a file with Comodo Antivirus (CIS5) and AV does not detect malware.
I have uploaded the file to “Virus Total”, and it shows:
Comodo found “Unclassified malware”.
My virus definitions are up to date.
What has happened?
Can somebody please help and answer me!!!
Thank you very much