Curve Cash Declined

I bought a cheap book on Amazon Kindle (£1.99), and the book downloaded fine.

However, I noticed later that I had an email from Amazon saying they couldn’t get the payment from my Curve card. No notifications on Curve. I tried a couple of times again using their payment link but no dice.

I then turned off “Use Automatically” for Curve Cash (balance was ~£2.50) and tried again and it worked.

Any ideas what was happening here?

PS. I also tried to explicitly set Curve to use the Curve Cash for payment and this was also declined without any notification.

Therefore, it seems I cannot pay with Curve using Curve Cash - to confirm, Curve works fine with a debit card selected.