Updated Date Problem

I wanted to show the latest updated date on my blog posts and Google SERPs.

So, I used [modified_date] tag in header element along with this PHP

function post_modified_date() {
    return get_the_modified_date();
}
add_shortcode( 'modified_date', 'post_modified_date' );

Everything worked perfectly, but i Don’t know why most of the page in Google SERP is not showing any date.

The page I need help with: [log in to see the link]