Outlook 2010 autocomplete not working.

The autocomplete function for outlook 2010 is where when you type a letter in the "To:" field, it gives you the history of email address to choose from according to the letter your typed in.

It was working for a while, I built up alot of email history. Then one day they all disappeared and it wont re populate again. Why is this nad what can I do to get it back.

The setting for autocomplete is turned on, btw.