EV Exchange Mailbox Task A1 Queue not going down

I have a problem whereby Enterprise Vault isn't archiving mail for the past week or so.

Having looked at the MS queues there appears to be 58,000 messages in the A1 queue.

The event log is littered with Event id 2216 and dtrace highlights an error such like "There is not enough Storage to complete this operation."

I am on EV 10.0.4 and have Exchange 2013 CU7 running on a Windows 2012 R2 server hosted on VM Ware.

To be honest things haven't really been the same since we moved from having Exchange 2007 on a physical server.