can not retrieve values from cache in custom module

I am building a module and I set the cache in the form function, which will be retrieved in the insert_new_email function.

I can retrieve the cached data in the form function, but I can not in the insert_new_email function.

Below is the class I use for my mod.email_blast.php file

class email_blast {

    public $return_data;

    /**
     * Constructor
     */
    public function __construct()
    {
        $this->EE =& get_instance();
        $this->EE->load->library('session');
        if (!class_exists('MCAPI'))
        {
            require PATH_THIRD.'email_blast/libraries/MCAPI.php';
        }
    }

    // ----------------------------------------------------------------

    public function form() {
        $service = $this->EE->TMPL->fetch_param('service', false);
        $api_key = $this->EE->TMPL->fetch_param('api_key', false);
        $list_id = $this->EE->TMPL->fetch_param('list_id', false);

        if($api_key == FALSE || $list_id == FALSE) {
            return "You are missing parameter(s)";
        }

        $this->EE->session->set_cache('email_blast', 'api_key', $api_key);
        $this->EE->session->set_cache('email_blast', 'list_id', $list_id);
        $this->EE->session->set_cache('email_blast', 'service', $service);

        // Build an array to hold the form's hidden fields
        $hidden_fields = array(
            'ACT'       => ee()->functions->fetch_action_id('Email_blast', 'insert_new_email'),
            'RET'       => ee()->functions->fetch_current_uri(),
            'API_KEY'   => $this->EE->session->cache('email_blast', 'api_key')
        );

        // Build an array with the form data
        $form_data = array(
            "id" => $this->EE->TMPL->form_id,
            "class" => $this->EE->TMPL->form_class,
            "hidden_fields" => $hidden_fields,
        );

        // Fetch contents of the tag pair, ie, the form contents
        $tagdata = $this->EE->TMPL->tagdata;

        $form = $this->EE->functions->form_declaration($form_data) . 
            $tagdata . "</form>";

        return $form;
    } 

    function insert_new_email()
    {
        $email = ee()->input->get_post('email');
        $email = trim(strip_tags($email));
        $api_key = $this->EE->session->cache('email_blast', 'api_key');
        $api = new MCAPI($api_key);
        $merge_vars = array ("OPTINIP" => $_SERVER['REMOTE_ADDR']);
        $list_id = $this->EE->session->cache('email_blast', 'list_id');
        $result = $api->listSubscribe($list_id, $email, $merge_vars, 'html', 'false', 'true', 'false', 'true');

        $data = array(
            'title'     => 'Mailing list',
            'heading'   => 'Thank you',
            'content'   => $list_id,
            'link'      => array($_POST['RET'], 'Site name here'),
            'email'     => $email,
        );

        ee()->output->show_message($data);
    }
}