The plug-in works well for how it is intended. The only issue for users it seems is being able to get the plug-in to showcase on the front-end of the site so that the subtitle actually appears. The way this plug-in works is by creating an input field that when the post is saved and this field is filled out, it will generate a post_meta value of “subtitle” and the key being whatever you just filled in for the Subtitle.
Here’s how to fix that issue. When you are within the loop of the post, you must do a get_post_meta() query to get the actual value of the subtitle that this plug-in generates. I went ahead and echoed the subtitle value underneath my main title like so:
<h1 class="entry-title single-title" itemprop="headline"><?php the_title(); ?></h1>
<?php if ( get_post_meta( get_the_ID(), 'subtitle', true ) ) { ?>
<h2 class="entry-title single-title" itemprop="sub-headline">
<?php echo $subtitle = get_post_meta( get_the_ID(), 'subtitle', true); ?>
</h2>
<?php } ?>