what i want to do is to redirect all visitors going to example.com to www.example.com.it seems a very common task but for some reason it is not working for this specific site .it always points to the default one. And strangely, if i replace the domain with another one(yyyyy.com and www.yyyyy.com), it works all right.i check my DNS,and it's resolved to the right IP.

here's my virtual host configure:

<VirtualHost *:80>
 ServerAdmin webmaster@localhost
 DocumentRoot /var/www/html/example.com
 Servername www.example.com

 <Directory />
  Options FollowSymLinks
   AllowOverride All
  </Directory>
<Directory /var/www/html/example.com>
  Options Indexes FollowSymLinks MultiViews
  AllowOverride All
  Order allow,deny
  allow from all
 </Directory>
</VirtualHost >

<VirtualHost *:80>
   ServerAdmin webmaster@localhost
   Servername example.com
   Redirect 301 / http://www.example.com
</VirtualHost>