Problem in outputting shortcode

This is my 2nd question about shortcode as I'm learning shortcode.

Here is my code:

add_shortcode('work-shortcode', 'work_shortcode');

function work_shortcode() 
{

$return_string .= '<div class="work">';
$return_string .= '<div class="container">';
$return_string .= '<div class="portfolio-wrapper">';
$return_string .= '<ul class="filter text-center wow fadeInUp" data-wow-delay="300ms">';
$return_string .= '<li>';
$return_string .= '<a class="selected" href="#" data-filter="*">All';
$return_string .= '</a>';
$return_string .= '</li>';

$terms = get_terms("filter"); // get all categories, but you can use any taxonomy
$count = count($terms); //How many are they?
if ( $count > 0 ){  //If there are more than 0 terms
    foreach ( $terms as $term ) {  //for each term:
        $return_string .= '<li>';
        $return_string .= '<a href="#" data-filter="'.$term->slug.'">';
        $return_string .=  $term->name;
        $return_string .= '</a>';
        $return_string .= '</li>';
        //create a list item with the current term slug for sorting, and name for label
    }
}
 $return_string .= '</ul>';

 $our_work = new WP_Query(array(
    'post_type' => 'our_work',
    'posts_per_page' =>50,


 )); //Check the WP_Query docs to see how you can limit which posts to display



if ( $our_work->have_posts() ) :

$return_string .= '<ul class="portfolio-items">';

while ( $our_work->have_posts() ) : $our_work->the_post();

    $termsArray = get_the_terms( $post->ID, "filter" );  //Get the terms for this particular item

    $termsString = ""; //initialize the string that will contain the terms

    foreach ( $termsArray as $term ) { // for each term 
        $termsString .= $term->slug.' '; //create a string that has all the slugs 
    }




    $return_string .= '<li class="'.$termsString. 'item">';
    $return_string .= '<div class="infos">';
    $return_string .= '<div class="img-responsive">';
    $return_string .= get_the_post_thumbnail();
    $return_string .= '</div>';
    $return_string .= '<div class="overlay">';
    $return_string .= '<div class="text">';
    $return_string .= '<h2>'.get_the_title().'</h2>';
    $return_string .= '</div>';
    $return_string .= '</div>';
    $return_string .= '</div>';
    $return_string .= '</li>';

    endwhile;

    wp_reset_postdata();

    $return_string .= '</ul>';
    $return_string .= '</div>';
    $return_string .= '</div>';

    endif;

return $return_string;
}

I'm having two problems actually:

1) My filters are not working(its working in page but not in shortcode). Here is code in page

<ul class="filter text-center wow fadeInUp" data-wow-delay="300ms">
   <li>
      <a class="selected" href="#" data-filter="*">All</a>
   </li>                         

<?php 
$terms = get_terms("filter"); // get all categories, but you can use any taxonomy
$count = count($terms); //How many are they?
if ( $count > 0 ){  //If there are more than 0 terms
foreach ( $terms as $term ) {  //for each term:
echo "<li><a href='#' data-filter='.".$term->slug."'>" . $term->name . "
</a></li>\n";
//create a list item with the current term slug for sorting, and name for label
}
} 
?>

</ul>

2) The shortcode not outputting the post thumbnail but when I put get_permalink() its outputting the links.. please help. Thankx..