Today I am exporting mail to PST so I can start consolidating my Exchange Servers and when running the New-MailboxExportRequest command you might find that the mailboxes just stay in a queued status without processing. You can check this by running the following command:
Get-MailboxExportRequest | Get-MailboxExportRequestStatistics
To fix the problem above simply open up Services.msc and restart the Microsoft Exchange Replication Service, give it a few min and if you run the above command you will see the Percentage Completed Column start showing you info instead of 0%.
Hope it helps