Can't add HSBC cards to Apple Pay on iPhone, but can on iPad

A few months ago I reset my iPhone 6 without deleting my cards from Apple Pay. Since then, I have been unable to re-add my HSBC debit and credit cards - I keep getting "Card Not Added - contact your issuer".

I have had no problems with adding cards from other banks. However, HSBC told me that because I hadn't deleted the cards properly before resetting my phone, their policy was that I had to wait 30 days before reatempting.

So after 30 days I tried again and it still didn't work. I have tried everything, from resetting all settings, to a fresh set up of the iPhone (not restoring from backup). I think the latter proves that this is related to the device and not my setup.

Then, the other day, I tried adding the HSBC cards to Apple Pay on my iPad and it worked instantly, so it's clearly nothing to do with my cards or bank account.

Can anyone suggest why my iPhone is refusing to allow me to add these particular cards to Apple Pay?