If you are hosting your website on Omnibasis. most likely it starts with www, for example www.mydomainname.com.
If customer enters mydomainname.com some browsers will redirect to www.mydomainname.com but not all.
You need to configure your DNS provider to redirect default domain to www.
Here are the links to help you out, if you do not see a link, search for "redirect default domain to www dns" with the name of your service provider.