Unable to purge recoverable items from a mailbox

Running the following command;

Get-MailboxFolderStatistics -Identity Mailbox -FolderScope RecoverableItems | Format-Table Name,FolderAndSubfolderSize,ItemsInFolderAndSubfolders -Auto

This produces the following results

Name              FolderAndSubfolderSize         ItemsInFolderAndSubfolders
----              ----------------------         --------------------------
Recoverable Items 3.148 GB (3,379,613,661 bytes)                      10548
Deletions         0 B (0 bytes)                                           0
Purges            0 B (0 bytes)                                           0
Versions          0 B (0 bytes)                                           0

I then run

Search-mailbox -identity mailbox -SearchDumpsterOnly -DeleteContent

But it has no effect

Name              FolderAndSubfolderSize         ItemsInFolderAndSubfolders
----              ----------------------         --------------------------
Recoverable Items 3.148 GB (3,379,613,661 bytes)                      10548
Deletions         0 B (0 bytes)                                           0
Purges            0 B (0 bytes)                                           0
Versions          0 B (0 bytes)                                           0

We are running Exchange 2010 SP1 RU6