We have set up a scheduled task to run weekly once in a clustered environment. But we the job did not run during last week and we do not see anything in the server logs.
We performed a staggered restart this week after which the job started running properly.

Is this discrepancy due to 6.5 version, how can i get more information on the cause of this issue?

well there were some issues with schedulers not running properly … but make sure that u have not have schedulers threads running kind of situations …without any error/entry in server log its kinda hard to figure out what went wrong there