Hey MG-Stockport,
If you haven’t already, try restarting your iPhone. Restarting can often help with issues like the one you’re describing. Here’s how:
1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
Restart your iPhone
Also, testing adding the pass over a different network to further isolate the issue.
You can read more about adding a pass to Wallet, here:
Add, use, and share boarding passes, tickets, and other passes in Apple Wallet
Take care.