IF NOT EXISTS statement not working correctly after 1st execution

I am trying to insert data into a table feed.author from another table feed.tempXML. The First time I execute this statement it works perfectly fine, but when i add data in feed.tempXML and run it again the new authors are not being added into the feed.author table. Does the IF statement need to change ?

Code:

IF NOT EXISTS(SELECT  DISTINCT t.author FROM feed.tempXML t 
    JOIN feed.author a ON a.author = t.author)
        INSERT INTO feed.author(author)
        SELECT  DISTINCT author
        FROM feed.tempXML
        WHERE author  IS NOT NULL