When joining a new cluster member to an existing Symantec Encryption Management Server installation, the existing Web Email Protection messages are not synchronized to the new cluster member even though the "Message Replication" setting is set to "All" in the Web Email Protection settings.
To work around this issue, activate the Message Replication setting before joining the member to the cluster. To do this:
Newly created messages are also synchronized corrrectly, thus if you have a short Message Expiration time and the cluster is already set up, you could just prevent traffic from reaching the newly added cluster member until the Message Expiration timeframe has passed (i.e. all existing messages were created after the cluster member was added).