Hello,
What version of Symantec Endpoint Protection are you using?
As you said, that Symantec is not detecting the Threat and the Other Antivirus software is detecting it, could you tell us if the Threat was detected and Removed OR it has not taken any steps till now?
Incase, if it has not removed the Files, I would always recommend you to follow the given steps in the Article and send the Suspicious files collected from the Machine by Symantec Support Tool to be Submitted to the Symantec Security Response Team.
Using Symantec Support Tool, how do we Collect the Suspicious Files and Submit the same to Symantec Security Response Team.
https://www-secure.symantec.com/connect/articles/using-symantec-support-tool-how-do-we-collect-suspicious-files-and-submit-same-symantec-sec
Scanning a file with a competitor's antivirus program detects a virus, but scanning with Symantec AntiVirus or Symantec Endpoint Protection does not
http://www.symantec.com/business/support/index?page=content&id=TECH98929&actp=search&viewlocale=en_US&searchid=1300105600178