Do you have Notes in Outlook? If you do, make sure they all have subjects.
See this for more Notes syncing info -> iOS: Syncing Notes
Unable to sync notes with Outlook
If notes are not syncing from Outlook, verify that you do not have any notes with no content or subject in Outlook. These notes can prevent Outlook from syncing as expected; removing them or adding a subject should allow syncing to resume.
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More information read this blog: http://diskdatarecovery.jimdo.com/2013/01/23/migrate-and-access-lotus-notes-nsf-files-in-ms-outlook/
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