There are a few reasons why this could be happening:
If it isn’t included, then you would need to add the following line to your theme’s footer.php file, immediately before the closing body tag.
<?php wp_footer(); ?>
3) A security plugin is blocking the plugin from loading the data
If you have a security plugin installed on your site then there’s a small chance that it may be causing the plugin to receive a 403 Forbidden Error when trying to load the PHP file it uses to retrieve the number of likes and comments from Facebook. This could be due to file permission settings that the security plugins is applying to certain directories or files on your site. Try temporarily disabling the security plugin to see whether this is the cause of the issue.