Hello wiffysmiffy1994.
Thank you for using Apple Support Communities. It sounds like you're experiencing issues with your iPhone. Specifically, your Apple Pay cards are not displaying when double-tapping the side button. We can understand how this would be an issue. Allow us to investigate this issue and assist in any way we can.
You did mention that this issue is only occurring with your iPhone and not your other devices that use Apple Pay. First, you can try a basic step, which is to restart your iPhone:
Restart your iPhone - Apple Support
If this step doesn't help, you might want to try removing the cards that will not display properly and re-add them to the Wallet app:
Manage the cards that you use with Apple Pay - Apple Support
Set up Apple Pay - Apple Support
Additionally, you'll want to make sure your iPhone is fully updated as well. If you have any pending updates, it would be best to create a backup and install that update. Keeping your device fully updated can help resolve various issues and help with overall performance. Just in case, the resources below can guide you:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
If these steps don't help resolve your issue. Please reach out to Apple Support for further assistance. You can use the link below to reach them:
Get Support - Apple
Thank you for using Apple Support Communities. Cheers!