The best would be for you to upgrade the jboss portal and/or Jboss ASversion you are using as later versions of portal had few clustering fixes.
Clustering is kind of setup where I would recommend you get support contract as they have hardware and environment to reproduce your problem.
Now, for your problem, looks like some sessions need to be flushed right away given that you are seeing replication at later stage.
Thanks for you reply. For clustering I am using TCP protocol instead of UDP. Is this causing the delay?Can this be a network issue or firewal setting causing the problem?