Simply renaming the NK2 file rarely works. Autocompletion is not a robust or
reliable feature. It frequently stops working usually because Outlook loses
its connection to its autocompletion cache. This can happen for a number of
reasons, most often because Outlook is not exiting completely and thus not
writing to the cache. Make sure you run no add-ins or sync software that
might hold Outlook open after you exit. After you find and resolve the
problem that is preventing Outlook from exiting properly, you may need to
create a new Outlook profile to get autocompletion to work again. If you're
lucky you'll be able to migrate your old cache to the new profile following
directions here:
http://www.ingressor.com/autocompletetips.htm