add_filter the_content in functions.php not working

This part of the single post template is called the loop:

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

Within the loop you need to call in the content:

<?php the_content(); ?>

Replace your single post template with this and see if it works now:

<?php get_header(); ?>      
<div id="content" class="clearfix row">
    <div id="main" class="col-sm-8 clearfix" role="main">
        <?php if (have_posts()) : while (have_posts()) : the_post(); ?>

        <article id="post-<?php the_ID(); ?>" <?php post_class('clearfix'); ?> role="article" itemscope itemtype="http://schema.org/BlogPosting">

            <header>



                <div class="page-header clip_content single-post">
                                <div class="page-header">
                                    <h1 class="single-title-wg" itemprop="headline">
                                        <?php $category = get_the_category();
                                        if ($category[0]) {
                                          echo '<b>'.$category[0]->cat_name.' '.$post->ID.'</b>';
                                        }
                                        ?>
                                    </h1>
                                    <h4 class="single-title" itemprop="headline">
                                    <p class="lead"> <?php the_title(); ?></p></h4>
                                    <p><?php the_content(); ?></p>

                                </div>


    <div class="item_footer">
                                            </div>            
                    <?php $category = get_the_category();
                    if ($category[0]) {?>
                    <p class="authorParagraph">
                        <?php echo '<a href="' . get_category_link($category[0]->term_id) . '" class="clearfix">' . $category[0]->cat_name . '</a></p>';
                        }
                        ?>
                </div>
                </div>

            </header> <!-- end article header -->

            <footer>
                <!-- <?php the_tags('<p class="tags"><span class="tags-title">' . __("Tags","wpbootstrap") . ':</span> ', ' ', '</p>'); ?> -->
            </footer> <!-- end article footer -->

        </article> <!-- end article -->



        <?php endwhile; ?>          

        <?php else : ?>

        <article id="post-not-found">
            <header>
                <h1><?php _e("Not Found", "wpbootstrap"); ?></h1>
            </header>
            <section class="post_content">
                <p><?php _e("Sorry, but the requested resource was not found on this site.", "wpbootstrap"); ?></p>
            </section>
            <footer>
            </footer>
        </article>

        <?php endif; ?>

    </div> <!-- end #main -->

</div> <!-- end #content -->

<?php get_footer(); ?>