Post loop is uneven

I’m trying to make even post loop using bootstrap but some of the rows are uneven.

This is my website https://salon.x3.rs/

Here is the code:

<?php if(have_posts()){
									while(have_posts()){the_post();
										
										?>
										<!-- Post -->
										 
										<article class="col-md-6 col-sm-12 col-xs-12 post-border clearfix">
											<header>
											<a href="<?php the_permalink();?>" class="image featured">
											<img src="<?php
											if(has_post_thumbnail()){
												the_post_thumbnail('medium');
											}
											else{
												echo "NO THUMBNAIL";
											}
											
											?></a>
											<div class="inner">
											<div class="row">
											<small  class="tag col-xs-3"><?php the_category(','); ?></small>
											</div>
											<div class="caption">
												<h4 class="post-naslov"><a  href="<?php the_permalink();?>"><?php the_title();?></h3>
											<div class="meta">
												<span class="date"><?php the_time('d M Y G:i'); ?></span>
										 
												 
											</div>
											</div>
											</header>
									 
										</article>
									 
 <!-- Post -->
 <?php
									}