It could be when the internet connection is weak or not existing.

When you show the barcode for the first time on any travel day, you need internet. Therefore, it's best to show it immediately after adding your first journey of the day to your pass.

For any following occurrences on a travel day you want to show the barcode, you don't need internet but without internet or with a weak connection, it can take several minutes to open. In such as case, it helps to switch your phone to airplane mode.

Try to switch to flight mode. If the internet connection is bad the app might play tricks.

Have you checked that you have the latest version of the app?

Thanks for the replies. I usually have my phone on flight mode. The version of the app I have is 5th September which I guess must be the latest one. This problem only started today, and I haven't been able to open the ticket at all although I have tried numerous times.

Your phone must be online at least every 72 hours. Otherwise your pass will not generate any QR codes.