is_single() dose not show thumbnails

I am using get_template_part(‘content’); in single.php. The the_post_thumbnail(‘large-thumbnail’); function shows the thumbnail image perfectly before. But it does not show with is_single() function in content.php.

<?php
if(is_single()){
the_post_thumbnail('large-thumbnail');
the_content();
}else{
the_post_thumbnail('small-thumbnail');
echo get_the_excerpt();
?>
<a>">Read more&raquo;</a>
<?php } ?>

single.php:

<article>
        <div class="single_post_content">
            <h5><?php the_title(); ?></h5>

            <?php
            get_template_part('content');
            ?>
        </div>
    </article>

• This topic was modified 7 years, 10 months ago by Steven Stern (sterndata). Reason: put code in backticks, moved to "developing wordpress"
• This topic was modified 7 years, 10 months ago by Steven Stern (sterndata).