If you had recently only changed nameservers on the domain it can take up to 48 hours for the DNS to propogate and it's not uncommon for the www and non www to resolve to different hosts. For example if I change nameservers from one parking company to another for a period the www and non www can resolve to DIFFERENT parking companies.
I think you just need to have waited a little longer
The .htaccess suggestion above is also worth doing if you have access to it so that there is only ONE site not TWO for SEO purposes