This can happen when:
- You are not receiving notifications. To make sure that you receive notifications, refer to Why is the order status in Magento not being updated?
- The shopper closed the browser before completing the payment. In this case, the offer in your Adyen Customer Area will automatically expire. By default, you do not receive a notification after the offer has expired, which is why the order status in Magento remains PENDING. If you want such orders to be automatically cancelled in Magento, contact Support to enable OFFER_CLOSED notifications.