You may see error like (#551) This person isn't available right now in report of Subscriber Bulk Broadcast with Messenger Broadcaster addon
There may several reason:
1. The user deactivated their account.
2.The user blocked your page.
3. The user don't have activity for long days with your page.
4. The user may in FB Inboxer leads as got private reply of comment but never replied back.
5. You don't have page messaging approval of your app.
There may any reason of them. And with BOT API they are not applicable to be BOT Subscriber. If you update through cron job, system will delete these kind of leads.
So make sure to set the cron job for updating the migrated list. It inside Messenger BOT Cron Job Menu.