WordPress wrong ajax response

I have a questio related with ajax. Sometimes it gets the right response but sometimes just returns 0. What could be the problem? Host or code?

Code is below:

jquery:

$(document).ready(function(){
    //var cat =  $(".bear_image").children("img").attr("alt");
    var top = $("li");
    var icons = $(top).children(".icons").children("div");

    $(icons).click(function(){
        //var icon = $(this).attr("class");
        var item = $(this).attr("id");
        var bear_id = $(this).parents("li").find("> .bear_image img");

        var data = {
            'action' : 'get_image_name',
            'item' : item,
            'bear_id' : bear_id.attr('alt')
        };

        $.post(ajaxurl, data, function(response) {
            //alert('Got response from server: ' + response);
            $(bear_id).attr("src", response);
        });
    }); 
});

functions.php

function jj_get_images()
{
    global $wpdb; 

    $item = esc_attr($_POST['item']); 
    $bear_id = esc_attr($_POST['bear_id']); 
    $query = "SELECT * FROM wp_acc_img WHERE position = " . $item . " AND bearid = ". $bear_id;

    $result = $wpdb->get_row($query);
    $c = $result->num_rows;
    $upload_dir = wp_upload_dir();
    $var = $upload_dir['url'] . "/" .$result->img.".png";

    die($var);

    wp_die();


}
add_action('wp_ajax_get_image_name', 'jj_get_images');



function pluginname_ajaxurl() {
?>
<script type="text/javascript">
var ajaxurl = '<?php echo admin_url('admin-ajax.php'); ?>';
</script>
<?php
}
add_action('wp_head','pluginname_ajaxurl');

It seems okay for me. The thing is that sometimes it return the full url that is how should it be but sometimes it just returns 0 and does not execute the code properly? Do you have any suggestions.

Thanks for the help 🙂