cannot get data from multiple categories

I have this function

$args = array ( 'cat' => '29,30,31,32', 'orderby' => 'rand', 'posts_per_page' => '1' );
    $the_query = new WP_Query( $args );
    if ( $the_query->have_posts() ) :
        while ( $the_query->have_posts() ) :
            $the_query->the_post();
            echo '<a href="'.get_the_permalink().'">'.get_the_post_thumbnail().'</a><br />';
        endwhile;
    endif;
    wp_reset_postdata();

It seems running correctly with one record is returned. But somehow, the link and the thumbnail returns nothing. If I replace the ‘cat’ with just one value, instead of 4 values, it’s working.

Am I wrong somewhere? Please suggest.

Thanks.