How to show children comments in wordpress when reply

Im working on a project in WP
for the comments list i used a callback the codes are here:

    <?php
function moortak_comment($comment, $args, $depth) {
   $GLOBALS['comment'] = $comment; ?>
   <li class="media" id="li-comment-<?php comment_ID() ?>">
        <div <?php comment_class(); ?> id="comment-<?php comment_ID(); ?>">
            <a class="media-left" href="<?php echo htmlspecialchars( get_comment_link( $comment->comment_ID ) ) ?>" title="<?php printf(__('%1$s at %2$s'), get_comment_date(),  get_comment_time()) ?>">
                <?php echo get_avatar($comment,$size='64' ); ?>
            </a>
            <?php if ($comment->comment_approved == '0') : ?>
                <div class="alert alert-info"><?php _e('Your comment is awaiting moderation.') ?></div>
                <br />
            <?php endif; ?>
            <div class="media-body">
                <h4 class="media-heading"><?php printf(__('<cite class="fn">%s</cite> <span class="says">says:</span>'), get_comment_author_link()) ?></h4>
                <?php comment_text() ?>
            </div>
            <div class="metadata" style=" margin: 10px auto;">
                <?php comment_reply_link(array_merge( $args, array('depth' => $depth, 'max_depth' => $args['max_depth']))) ?>
                <?php edit_comment_link(__('(Edit)'),'  ','') ?>
            </div>
            <div class="clearfix"></div>
        </div>
<?php
        }
?>

now when I click on the reply button the list of the children comments aren’t show .

This is the comments.php codes:

    <?php
    if (!empty($_SERVER['SCRIPT_FILENAME']) && 'comments.php' == basename($_SERVER['SCRIPT_FILENAME']))
        die ('Please do not load this page directly. Thanks!');

    if ( post_password_required() ) { ?>
        This post is password protected. Enter the password to view comments.
    <?php
        return;
    }
?>

<?php if ( have_comments() ) : ?>
    <div class="responses">
    <h2 id="response-title"><?php comments_number('No Responses', 'One Response', '% Responses' );?></h2>
    <ul class="media-list" id="comments-list">
        <?php wp_list_comments("callback=moortak_comment"); ?>
    </ul>
    <div class="navigation">
        <div class="next-posts"><?php previous_comments_link() ?></div>
        <div class="prev-posts"><?php next_comments_link() ?></div>
    </div>
 <?php else : // this is displayed if there are no comments so far ?>

    <?php if ( comments_open() ) : ?>
        <!-- If comments are open, but there are no comments. -->

     <?php else : // comments are closed ?>
        <span class="alert alert-danger">Comments are closed.</span>

    <?php endif; ?>

<?php endif; ?>

<?php if ( comments_open() ) : ?>

<div id="respond" class="col-md-12" style="margin: 15px auto">
    <h2><?php comment_form_title( 'Leave a Reply', 'Leave a Reply to %s' ); ?></h2>
    <?php if ( get_option('comment_registration') && !is_user_logged_in() ) : ?>
        <p>You must be <a href="<?php echo wp_login_url( get_permalink() ); ?>">logged in</a> to post a comment.</p>
    <?php else : ?>

    <form action="<?php echo get_option('siteurl'); ?>/wp-comments-post.php" method="post" id="commentform">
        <?php if ( is_user_logged_in() ) : ?>
            <p>Logged in as <a href="<?php echo get_option('siteurl'); ?>/wp-admin/profile.php"><?php echo $user_identity; ?></a>. <a href="<?php echo wp_logout_url(get_permalink()); ?>" title="Log out of this account">Log out &raquo;</a></p>

        <?php else : ?>
        <?php
        $req      = get_option( 'require_name_email' );
        $aria_req = ( $req ? " aria-required='true'" : '' );
        $html5    = current_theme_supports( 'html5', 'comment-form' ) ? 1 : 0;
        ?>

                    <?php echo'<div class="form-group comment-form-author">' . '<label for="author">' . __( 'Name' ) . ( $req ? ' <span class="required">*</span>' : '' ) . '</label> ' .
                    '<input class="form-control" id="author" name="author" type="text" value="' . esc_attr( $commenter['comment_author'] ) . '" size="30"' . $aria_req . ' /></div>
                    ';?>

                    <?php echo'<div class="form-group comment-form-email"><label for="email">' . __( 'Email' ) . ( $req ? ' <span class="required">*</span>' : '' ) . '</label> ' .
                    '<input class="form-control" id="email" name="email" ' . ( $html5 ? 'type="email"' : 'type="text"' ) . ' value="' . esc_attr(  $commenter['comment_author_email'] ) . '" size="30"' . $aria_req . ' /></div>'
                    ;?>
                    <?php echo'<div class="form-group comment-form-url"><label for="url">' . __( 'Website' ) . '</label> ' .
                    '<input class="form-control" id="url" name="url" ' . ( $html5 ? 'type="url"' : 'type="text"' ) . ' value="' . esc_attr( $commenter['comment_author_url'] ) . '" size="30" /></div>
                    ';?>
        <?php endif; ?>

        <div class="alert alert-warning">You can use these tags: <code><?php echo allowed_tags(); ?></code></div>

        <?php echo'<div class="form-group comment-form-comment">
            <label for="comment">' . _x( 'Comment', 'noun' ) . '</label>
            <textarea class="form-control" id="comment" name="comment" cols="45" rows="8" aria-required="true"></textarea>
        </div>'; ?>

            <?php echo '<button class="btn btn-success" type="submit">' . __( 'Submit' ) . '</button>';?>
            <?php comment_id_fields(); ?>
            <?php cancel_comment_reply_link('<span class="btn btn-danger">Cancel Reply </span>'); ?>
            <?php do_action('comment_form', $post->ID); ?>

    </form>

    <?php endif; // If registration required and not logged in ?>

</div>
<div class="clearfix"></div>
</div>
<?php endif; ?>

When I Use Inspect Elements It shows the code but not showing in the screen.

also if possible inform a reference for full customizing CSS comment list.

Thanks alot for your attention.