Sounds like for whatever reason the IPN got interrupted and CBSubs wasn't notified of the payment. This can happen if your webhost server goes down for a few minutes, or right when the IPN is sent, or the Worldpay server failed to send the IPN, or was down. Could've also been a fatal error on your site that stopped it. Was there no notification in CBSubs > Notifications for the purchase? What about any errors in CBSubs > History?