Hi,
1) Delete all older definitions on SEPM server.
2) Please run the Liveupdate from the SEPM.
For the same you may have to go to Admin > Server > Highlight Local Site> Click on Download Liveupdate Content
3) Please check that youe SEPM server having free disk space or not.
4) Make sure that you have installed latest version of live update on the SEPM.
5) Even i would like to request please Migrate the SEPM to the Latest version of SEP 12.1.2015 (RU2)
Please check with these Articles:
Symantec Endpoint Protection Manager (SEPM) does not update virus definitions. Liveupdate not working on the Symantec Endpoint Protection Manager 12.1
http://www.symantec.com/docs/TECH183178
And if you having proxy configured then check below article.
How to Configure the Proxy settings in the Symantec Endpoint Protection Manager (SEPM) 12.1
https://www-secure.symantec.com/connect/articles/how-configure-proxy-settings-symantec-endpoint-protection-manager-sepm-121
Required exclusions for proxy servers to allow Symantec Endpoint Protection to connect to Symantec reputation and licensing servers
http://www.symantec.com/docs/TECH162286
http://www.symantec.com/docs/TECH183178
https://www-secure.symantec.com/connect/articles/t...
http://www.symantec.com/business/support/index?page=content&id=TECH183178
Hope this may help you!!!