Loss of responsiveness when forwarding with masking

Like T3 and have built a nice site with it. Have just forwarded the old .com address to the new .co.uk address where we had built the new site.

Works fine. Accept if I do "forwarding with masking" the site is presented as a standard web page and is no longer reponsive!

Is this a T3 issue or a Joomla Issue?

I want responcive but I also want to mask the .co.uk with the .com? But I cant have both at the moment.

Many thanks
Colin