In one case, which may or may not be like yours, having an alias account kept the migration from going through. Once the alias account was deleted the migration went through, BUT the deleted alias account was NOT able to be recreated. From what I understand, an alias account is NOT the same as a sub-account. Thought I'd mention that because your question does address sub-accounts. This migration sure is a pain, IMO.
If this is no help, you can read some of the google sites from the above link to see if you can resolve this b4 AT&T responds.