In Outlook 2013 if you are doing an export ensure you have configured the profile to cache all the mailbox data and not just the last 12 months, which is the default;
However its easier to do an export from the server side using Powershell;
Create a new share with the following permissions;
- Share permissions - Everyone F/C
- NTFS permissions - Everyone F/C + Exchange Trusted Subsystem F/C
Open the Exchange Shell as an Administrator and type;
New-MailboxExportRequest -Mailbox "user" -FilePath "\\uncpath.pst"
To check progress;