When you add a custom size, to use it as a featured image, your template needs to specifically pass this size in a featured image call. For example the_post_thumbnail('my-new-thumb');. Unless you’ve changed your templates, they are still using the default “post-thumbnail” size. I’m not sure how the regenerate thumbnails plugin actually works. It may only resize “post-thumbnail” sized images. IDK, you can check you image files to see what has changed.

If the 320×180 size is what you would like the default featured image size to be and you want to avoid editing your templates, use set_post_thumbnail_size() instead of add_image_size(). This will make the “post-thumbnail” size 320×180. When your template calls something like the_post_thumbnail() with no parameters, it’ll use that size.

You do need to set the size after your parent theme does or else the size will be overridden. assuming your code is in a child theme, its code executes first, so you would need to set the size in a callback for the ‘after_setup_theme’ action or later. It’s also possible your theme uses its own special featured image size, in which case the name would be passed in featured image calls like the_post_thumbnail('themes-special-size');. If so, either set the size for that name or edit the function calls.