This can occur if the vault store partition has been configured as a NetApp Device and the Device stores data in SnapLock mode option has been selected.  The Device Type for the partition will be displayed as NetApp Device [SnapLock] when viewed in the Vault Administration Console (VAC).  When attempting to delete an archive the following warning will be seen in the Enterprise Vault (EV) Event Viewer log:

Type: Warning
Event: 7113
Source: Enterprise Vault
Category: Storage Delete
Computer: EVSERVER
Description:
Following archive could not be deleted as it contains items that are on hold, or their retention category does not allow deletions, or a compliance device retention period prevents them from being deleted.
Archive Name = EV, User
Archive Entry Id = 1FO93HG311E09D94AB04C3B8B34743F1F1110000evsite01
Total items on hold = 1066

The following will be seen in a Dtrace of the StorageDelete process when attempting to delete the archive.  For more information on Dtrace please refer to the link in the Related articles section.  

(StorageDelete) <6916> EV:M CVault::DeleteSaveset (Exit) |The deletion of the item is not currently permitted. Either the item is on hold, or its retention category does not allow deletions, or a compliance device retention period prevents it from being deleted.   [0xc0041a57] |
(StorageDelete) <6916> EV:L CDeleteVault::DeleteSavesets Information: Saveset [entryId = 1663B28D2A2C4B0D8E63224C4172EC70] can't be deleted as it is on hold.

It is not possible to delete an archive until all the items in the archive have passed the retention period that has been set in EV.   Once all items in the archive have exceeded the retention level, the archive can be deleted.