You may need to regenerate your images. There are plugins to accomplish this or re-upload your image and see if the size doesn’t take affect. It won’t happen automatically to existing files.
First, check your uploads directory to see if the files exists that way you know they are being generated. Then try another function to output the results.
What do you end up with when you use:
$attachment_id = get_post_thumbnail_id( $post->ID );
$img_src = wp_get_attachment_image_url( $attachment_id, 'cover-size' );
$img_srcset = wp_get_attachment_image_srcset( $attachment_id, 'cover-size' );
Also if may be handy to output your meta data for that attachment – specifically to see the size information:
print_r ( wp_get_attachment_metadata( $attachment_id) );
The above wp_get_attachment_image_srcset requires WordPress 4.4 and you can find more information about Responsive Images in WordPress 4.4.
Assuming the files exist:
-
the_post_thumbnail
( $size, $attr ) -
get_the_post_thumbnail
( int|WP_Post $post = null, string|array $size="post-thumbnail", string|array $attr="" )
Are you in a Loop or outside of it?
If you’re inside try the_post_thumbnail ('cover-size');
otherwise echo get_the_post_thumbnail ($post,'cover-size');