When Alan mentioned DA he was referring to one of our acclerator products called discovery accelerator. So if you have this installed you can create searches of mail and place certain messages on legal hold. If you have messages on legal hold which are in the particular archive that you are trying to delete then that will stop your archive from being deleted.
If you do not have DA installed then it cannot be this. What storage are you using as it could be centera retention playing a part here if you have the data stored on a centera.
If you are on NTFS then what I would do is reset your archive back to normal by changing the status in the archive table to go back to a normal state. Not in front of a system at the minute to check this but it will be obvious as all other archives will have the same value (0 I think off top of head) and the one as marked for deletion will have a different value.
Then what youu can do is setup a dtrace of the deletion process and attempt the delete again and this information to the trained eye should indicate where the deletion process is stopping. Again off top of my head I cannot remember the process but storagedelete is probably the process to dtrace.
If you are not comfortable with any of the above procedures then I would receommend opening a support case.
