add_image_size not working

I have this code on my theme functions.php:

function theme_initiated() {
	add_theme_support( 'post-thumbnails' );
	add_image_size( 'custom_size', 980, 730, true );
}
add_action( 'after_setup_theme', 'theme_initiated' );

And this on index.php

<?php the_post_thumbnail( 'custom_size' ); ?>

However, it does not crop the image size, instead, it just show full size image.

This only happen on 4.6.1 version.

Any help would be appreciated.

Thanks!