How to query custom post types posts filtered by multiple custom taxonomies through a form selection

Apparently the problem was in the “AND” relation. Code now seems to work:

 <?php    
     if(isset($_REQUEST['submit']))
                { 
                    $prodotto = $_POST["prodotto"];       
                    $livello = $_POST["livello"];       
                    $ruolo = $_POST["ruolo"];       
                    $stato = $_POST["stato"]; 



                    $args = array( 
                            'post_type' => 'corsi',
                            'posts_per_page' => -1,
                            'orderby' => 'title',
                            'order'   => 'ASC',
                            'tax_query' => array(

                                array(
                                    'taxonomy' => 'prodotto',
                                    'field' => 'slug',
                                    'terms' => $prodotto,
                                         ),
                                array(
                                    'taxonomy' => 'livello',
                                    'field' => 'slug',
                                    'terms' => $livello,
                                         ),
                                 array(
                                    'taxonomy' => 'ruolo',
                                    'field' => 'slug',
                                    'terms' => $ruolo,
                                         ),
                              array(
                                    'taxonomy' => 'stato',
                                    'field' => 'slug',
                                    'terms' => $stato,
                                         ),


                                      ) 
                                    );
                    $soluzioni = new WP_Query($args);

                                if ( $soluzioni->have_posts() ) {
                                        echo '<ul>';
                                            while ( $soluzioni->have_posts() ) {
                                                $soluzioni->the_post();
                                                                    echo '<li>' . get_the_title() . '</li>';
                                                                            }
                                                        echo '</ul>';

                                                            wp_reset_postdata();
                                                                } else 
                                                                    {
                                                                           echo '<br/> OPPALLA no post';
                                                                    }




                             }