I've had this happen from time to time on the website. You can try logging out and back in or using a different browser but I've never found a guaranteed solution. Both the website and the app have their own problems (for instance, I don't believe it's possible to apply a credit shell or voucher in the app), but typically I book on the app unless there's a case where I need to book on the website. The website feels like it's held together with duct tape and glue.