No shipping options for some countries

I'm using Opencart 3.0.2.0 and Royal Mail shipping. When I test "estimate shipping" in the cart, some countries get the "no shipping options" but not others.

For example Turkey is fine Switzerland is not. I can apparently ship a product to the Faroes Islands but not the same product to the Falklands Islands (Malvinas).

All the counties are in the geo zones. The weights are grammes throughout and Royal Mail General is set to All Zones. The country codes match.

Any idea why I can ship the same product to one country, but not another, please?

Can anyone help? I'm really stuck for an answer.

Thanks

Graham