The last time I did an IMAP mailbox migration there was such a delay that I tried to rerun it 6 times, each time appearing like it completed but nothing was there in the Inbox. Apparently it was still queued and in process (in the end I got 7 copies of each of 546,000 messages, turning a 15GB mailbox into a 105GB mailbox!).
If the mailbox being imported is significantly large it can take quite a long while before it completes (for me it took 12+ hours for the mailbox mentioned above. The hundreds of IMAP mailboxes I migrated before that one were pretty quick and only took 5-10 minutes). But now that you mentioned it, the last one that took a long time was also a Dovecot IMAP Server as well. I'm wondering if that can be an issue, not just the mailbox size.
Have you looked to see if there are still items in the \Smartermail\IMAP Retrieval\ folder? If so, then the import is still in process.