This issue is usually caused by the web hosting not including the WWW. prefix (the tick box must be ticked when creating the domain for WWW. to work correctly). To resolve this on an existing domain login to Plesk and follow the below steps:
First click the Domains option on the left hand menu to bring up the list of configured domains on your VPS/Server.
Next tick the tick box next to the domain that doesn't work when using WWW. and click the Modify option from the tool bar just above the domains list.
You will see a list of radio options on the new page one of which will be labelled WWW. prefix. Tick the radio option to on for this option then click the Save button at the bottom of the page. You should now see your website content instead of the default Plesk page when browsing to the domain with the WWW. prefix
If this has still not resolved the issue please submit an enquiry via the Namesco Online Control Panel.