It's something which is often talked about on the Symantec Connect Forums... that's right, stuck messages in the journal mailbox. There are many reasons why this might be happening. One of them I just spotted in a new Symantec Enterprise Vault technote.
http://www.symantec.com/docs/TECH206025
The issue described in the technote is that sometimes mass-mailers generate messages all with the same message-id. It's just 'easier' for them, or poor coding, you can decide which! The Enterprise Vault journal task does it's grouping of messages to process in the journal mailbox using message-id, so you can see that having many that are the same, but with different messages, or to different people might cause some issues.
Unfortunately the technote doesn't give any fix, solution or workaround, so it's one that you should probably subscribe to and see when it gets updated. Are you affected by this type of issue?