Can’t clear items from “/Deleted Items”

I have a user that is reaching the mailbox limits set by the organisation.  When I check in Outlook and OWA  at the user, the items that are held in the deleted items and recover deleted items are negligible.  Yet, when I run Get-MailboxStatistics  User.Name | select DisplayName,TotalDeletedItemSize It returns…

DisplayName                                                 TotalDeletedItemSize
User.Name                                                 12.96 MB (13,594,195 bytes)

I then ran Search-mailbox -identity User.Name -SearchDumpsterOnly -DeleteContent

After confirming the action I then get this:
Identity         : User.Name
TargetMailbox    :
TargetPSTFile    :
Success          : True
TargetFolder     :
ResultItemsCount : 0
ResultItemsSize  : 0 B (0 bytes)

I then run the stats again and it returns:
DisplayName                                                 TotalDeletedItemSize
Mark Jacobs                                                 12.96 MB (13,594,195 bytes)

I have also waited for the retain deleted items time (RetainDeletedItemsFor : 15.00:00:00)to expire  and this doesn’t make any difference.

I have also run a repair against the mailbox.
There are no retention policies in place.
We run Exchange 2010 SP2 RU3.

Any ideas how to clear the deleted items?