Backing up Exchange 2010 DAG

I'm having difficulties backing up my Exchange 2010 DAG in Symantec 2012.

I've addded the DAG as a 'server' no problem, it finds both Exchange servers and installs the Symantec remote agent.

When choosing the DAG in the list of servers to create a backup, I'm unable to select it in the backup selections tab. It whirls for a while with the message 'querying for data' then does nothing. 

I've tried removing the DAG from the list of servers and re-adding it, ensuring the credentials are correct and re-installing the remote agent but nothing makes a difference.

Anyone have any ideas?

Tom