Attach another user mailbox - 'cannot expand the folder'

Hi,

We recently moved all our user mailboxes to exchange 2013 from exchange 2007. I used to be able to attach another users mailbox.

Now when I attach I am presented with error message 'Cannot expand the folder'

I am an exchange organisation admin and I have 'full access' to the users mailbox in the mail delegation section. However I get this message.

I want access to attach ALL users mailbox. What do I need to do to get this working?

Thanks