If you check the Eventlog, do you find these events?

The MAPI call ‘OpenMsgStore’ failed with the following error:
The information store could not be opened.
The MAPI provider failed.
MAPI 1.0
ID no: 8004011d-0289-00000000

For more information, click http://www.microsoft.com/contentredirect.asp.

Failed to open mailbox ‘/o=CONTOSO/ou=First Administrative Group/cn=Recipients/cn=JohnDoe’ in mailbox store ‘/o=CONTOSO/ou=First Administrative Group/cn=Configuration/cn=Servers/cn=ContosoMailServer/cn=Microsoft Private MDB81234567′ on server ‘ContosoMailServer’.
Error: The information store could not be opened.
The MAPI provider failed.
MAPI 1.0
ID no: 8004011d-0289-00000000

For more information, click http://www.microsoft.com/contentredirect.asp.

Quickly check if these users are not disabled.  Mailboxes with disabled users as associated account cannot be moved.  The workarounds are to enable the user accounts – which is not that desireable for your company’s Security staff – or assign SELF as the associated account.

More info at Microsoft.