Sub-theme template files not read

I've copied Sasson's templates directory to the sub-theme, then made some changes to page.tpl.php or html.tpl.php but those changes are ignored by the sub-theme, even after I save the sub-theme to update the theme registry.

How do I let my Sasson sub-theme know that the new template files exist? Or that I made changes to them?

Thanks

Amnon