I don't think that is a feature of www vs. no www. The choices are to add www, remove www, or allow both. It should always work without the www, but the panel allows you to control what happens when someone leaves off the www.
If it doesn't clear itself up, drop a line to support. They may have to refresh the DNS table. While you're at it, go to that masking domain's DNS and see what's listed. The non-www IP address (A Record) should be the same as the www. address.