Domain Mapping - sites with www. in front of URL does not open, sites without www. does open

Hi there,
I have a very strange situation going on

All sites on my network open when I only type the domain name in the Address Bar like :

thetophouse.co.za

but it doesn't open when I type in

http://www.thetophouse.co.za

I'm running an updated WP, with all plugins and themes updated.

my htaccess file looks like this :

RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*.php)$ $1 [L]
RewriteRule . index.php [L]

any urgent help advice will be appreciated very much