"New-MailboxImportRequest" failed?

I have one mailbox (9GB) which is on Exchange 2003 server.
I used Outlook 2007 to export to one PST file.
Then, I used "New-MailboxImportRequest" to import pst to
this user mailbox on Exchange 2010.

After about 800MB data has been imported, the mailboxImport failed
when I ran Get-MailboxImportRequest.

What did I do wrong? what caused import to fail?

Thank you.