Hello Srikanth,
Are these clients running on older SEP 11.x? If yes, they may face issues with definitions as the 11.x definitions are no more getting released.
If not, then in your case where there is no LUA / GUP in between, we may need your assistance by pulling sylink.log from any of machines which are not being updated.
http://www.symantec.com/docs/TECH104758
Secondly, are these machines all from the same group?
When was the last time you restarted the SEPM server?
Was there any change in proxy or firewall during these 10 - 15 day period?