problem with get_the_tags()

Hello, this get_the_tags() makes me crazy ??
I have the following php with loops:

<div id="filter-blog-isotope" class="portfolio-filter">
                            <div id="count-projects"><!--<i class="font-icon-forward">--></i><span class="current-number">00</span><!--<span class="num-name"> / Related Information</span>--></div>
                            <ul>
                                <li><a href="#" class="has-items selected" data-filter="*">All</a></li>                         

                                <?php

									//Define the loop based on arguments
                                    $loop = new WP_Query( $args );

                                    //Display the contents
                                    while ( $loop->have_posts() ) : $loop->the_post();   	

                                ?>                         

                                <li><a href="#" data-filter=".<?php
															  $posttags = get_the_tags();
															  if ($posttags) {
																foreach($posttags as $tag) {
																  echo $tag->slug;
																}
															  }
														  ?>"><?php
															  $posttags = get_the_tags();
															  if ($posttags) {
																foreach($posttags as $tag) {
																  echo $tag->slug;
																}
															  }
														  ?></a></li>                                   

                            	<?php endwhile;?>

                            </ul>
                        </div>                                  	

                        <div id="blog" class="wide">
							<div id="blog-posts-container" class="normal-blog-pagination">

								<?php

                                    //Define the loop based on arguments
                                    $loop = new WP_Query( $args );               

                                    //Display the contents
                                    while ( $loop->have_posts() ) : $loop->the_post();

                                    $url = wp_get_attachment_url( get_post_thumbnail_id($post->ID) );

                                ?>                         

                                <article class="item-blog <?php
															  $posttags = get_the_tags();
															  if ($posttags) {
																foreach($posttags as $tag) {
																  echo $tag->slug;
																}
															  }
														  ?>">

                                    <div class="post-container">

                                        <div class="post-creative post-image" style="background-image: url(<?php echo $url; ?>);">
                                            <a class="post-link" title="<?php the_title(); ?>" target="_blank" href="<?php the_permalink(); ?>">
                                                <div class="post-caption">
                                                    <div class="post-naming">
                                                        <h2 class="post-title"><?php the_title(); ?></h2>
                                                        <span class="post-date"><?php the_content(); ?></span>
                                                    </div>
                                                </div>
                                            </a>
                                        </div>
                                    </div>
                                </article>                                   

                            	<?php endwhile;?> 

                            </div>

                        </div>

trying to display a sortable article list. Works well except that the classes for my articles are retrieved (as expected) from the tags in each post but only for the first 3 posts (maybe because there are for now 3 tags defined). I have to have for the isotope li data-filter form the tags (which seems to work perfecter the first) and classes for the articles in reference to them. What am I doing wrong here?
Thanks so much for help!
Garavani