Bootstrap Article Loop

hey,

i have a problem with the loop and bootstrap 3.30 you can see on this picture.

my bootstrap article code is.

<?php
/**
 * The default template for displaying content
 */
?>
	<article class="col-xs-12 col-md-6 articles">
<?php    if ( has_post_thumbnail() ) {
echo "<div class=\"news-img\">" ;
the_post_thumbnail();
 echo "</div>"; } ?>
 <div class="white-content">
 <header class="article-headline-wrapper"><h2 class="article-headline"><?php the_title(); ?></h2></header>
 <br />
<div class="article-content"><?php the_content( $more_link_text , $strip_teaser ); ?><div class="post-data"><b><?php the_time("l, j. F Y"); ?></b></div></div>
</div>
	</article>

and the loop looks like this

<?php
/**
 * The main template file
 *
 * This is the most generic template file in a WordPress theme
 * and one of the two required files for a theme (the other being style.css).
 * It is used to display a page when nothing more specific matches a query.
 * For example, it puts together the home page when no home.php file exists.
 *
 */
get_header(); ?>
<div class="row">
<div class="col-xs-12 col-md-9" id="news-content"><h1 id="master-headline">News</h1>
			<?php /* Start the Loop */ ?>
			<?php while ( have_posts() ) : the_post(); ?>
				<?php get_template_part( 'content', get_post_format() ); ?>
			<?php endwhile; ?>
</div>
<nav class="col-xs-12 col-md-3" id="sub-nav">
<?php
if ( function_exists('dynamic_sidebar') && dynamic_sidebar('left_1') ); ?></nav>
</div>
<?php get_footer(); ?>

any ideas how to fix the spacings between the posts?