hi,
the possible reason why you are not able to expand the storage group under document level, the credentials required for expanding the same is that of a Exchange full administrator.
As the backups have been working fine for the document level, one location i would check is the properties of this user in active directory users & computers if it is exchange 2003
then on exchange server open active directory users & computers and check the advance properties of the user verify if the user account is hidden from Global address list. it would be a simple tick mark uncheck that and "bingo" you are there.
if this is exch2007 then you can verify the same in the exchange 2007 mgmt console under mail boxes highlight the user which is a exchange full admin and verify if this address is hidden from GAL and unhide it.
bhanu