The symptoms you describe are classic indicators of a problem that can happen with the Website Payments Standard module where the IPN notification sent from PayPal to your server doesn't get processed by your server ... either it's blocked and is not received, or encounters a problem processing what it does receive. Could be a problem at PayPal. Could be a problem at your hosting company. Could be a problem with your server. Could be a problem with addons you've installed. Could be a problem with .htaccess or security settings you've added. Could be a problem with things you've configured inside your PayPal account. Could be a problem with settings in your Zen Cart PayPal details.

Related article: http://www.zen-cart.com/wiki/index.php/PayPal