This is caused by a timing issue during the processing of the task that does not see the items as being ready for post-processing.
One of the following workarounds may help to post-process the items more quickly.
Workaround
1. The task can be run again immediately after the first run and the items should be post-processed without the need for a backup or replication to occur. This can be set with a schedule to process the task in two separate times per day to allow for the processing to occur.
Note: For tasks that take several passes to complete the scanning of the volume the post-processing would not occur until the volume has been completely scanned.
2. Use the FSARunNow command to process a ShortcutsOnly run on the task once the task is out of the scheduled run time. For more information on FSARunNow please refer to the Utilities Guide.
Enterprise Vault is working as designed in the scenario and items will be processed on the next pass of the volume.