checkbox issue with wordpress update_post_meta()

Hi,

usually it is a PHP issue but in my case i read on internet several posts about the way how to do it and it seems my code is correct, so i do not understand where is the problem.

I’m creating a plugin that should modify posts appearance.
For that i have a meta box where i have a checkbox and i want to store in DB the status of that check box (checked or not).
For that i use the following code:

/*
 * Function to render the meta box added by the plugin in post
 */
function rgt_metabox_callback( $post ) {

    // Add an nonce field so we can check for it later.
    wp_nonce_field( 'rgt_meta_box', 'rgt_meta_box_nonce' );

    /*
     * Use get_post_meta() to retrieve an existing value
     * from the database and use the value for the form.
     */
    $rgt_post_images_slider = get_post_meta( $post->ID, '_rgt_post_images_slider', true );
    $rgt_post_images_slider_usage = get_post_meta( $post->ID, '_rgt_post_images_slider_usage', true );

    echo '<label for="rgt_post_thumbnail_slider">';
    if($rgt_post_images_slider_usage == "true"){
        echo '<input type="checkbox" id="rgt_image_list_usage" name="rgt_image_list_usage" checked="checked" value="true" />';
    }
    else{
        echo '<input type="checkbox" id="rgt_image_list_usage" name="rgt_image_list_usage" value="true" />';
    }

    _e( 'Use images slider instead of Featured image.', 'rgt-post-thumbnail-slider-plugin' );
    echo '</label> ';

and later:

function rgt_save_meta_box_data( $post_id )
{

    /*
     * We need to verify this came from our screen and with proper authorization,
     * because the save_post action can be triggered at other times.
     */

    // Check if our nonce is set.
    if (!isset($_POST['rgt_meta_box_nonce'])) {
        return;
    }

    // Verify that the nonce is valid.
    if (!wp_verify_nonce($_POST['rgt_meta_box_nonce'], 'rgt_meta_box')) {
        return;
    }

    // If this is an autosave, our form has not been submitted, so we don't want to do anything.
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return;
    }

    // Check the user's permissions.
    if (isset($_POST['post_type']) && 'page' == $_POST['post_type']) {

        if (!current_user_can('edit_page', $post_id)) {
            return;
        }

    } else {

        if (!current_user_can('edit_post', $post_id)) {
            return;
        }
    }

    /* OK, it's safe for us to save the data now. */

    // Make sure that it is set.
    if (!isset($_POST['rgt_image_list'])) {
        return;
    }

    if (!isset($_POST['rgt_image_list_usage'])) {
        return;
    }

    // Sanitize user input.
    $my_data = sanitize_text_field($_POST['rgt_image_list']);

    $img_slider_usage = "";
    error_log("value of checkbox (in SAVE): ".$_POST["rgt_image_list_usage"]);

    if (isset($_POST["rgt_image_list_usage"])) {
        $img_slider_usage = $_POST['rgt_image_list_usage'];
    }

    // Update the meta field in the database.
    update_post_meta( $post_id, '_rgt_post_images_slider', $my_data );
    update_post_meta( $post_id, '_rgt_post_images_slider_usage', $img_slider_usage );
}
add_action( 'save_post', 'rgt_save_meta_box_data' );

However when checkbox is not checked, it does not clear value in DB.
So where is my mistake ?
thx