Hi Santosh
You need to check:
1) if the OU is assigned as Possible Agents in the task...PFTC->Additional Data->Agent Assignemnt maiantained
2) The Responsible agents -> How are the ageys assigned to the task? Via a rule or some custom code -> the OU should be there
The intersection of Possible and Responsible agents will result in Actual agents
Also check if there is any attribute of validity set for the OU.
regards,
Modak