From our experience we don’t alway rely on the information online so we raise enquiry with Royal Mail business customer services. In the past we found the customer informed us they had not received their order and the delivery information had not been updated on the website. I think there was something wrong with the handheld terminal? I am not saying that your customer isn’t telling the truth but sometimes better to check especially when its happened twice.
I would also recommend checking the delivery address as many use multiple delivery addresses.