Authorize.net not pinging WooCommerce

Hello! The website I’m writing about is http://www.henssgenhardware.com. We are using authorize.net for payments, but some transactions are marked “Pending” even though payment has been completed. For those transactions we are not being notified via email because the system is not recognizing the transaction as completed, even though it is. We called authorize.net and they said that everything is functioning properly on their end.