A few months ago, I ran into the same issue when trying to use a card for an online purchase. The transaction kept failing even though the balance was correct. I tracked down the right contact via https://vanilla-gift.pissedconsumer.com/customer-service.html and called during business hours. The agent explained that sometimes new cards need 24 hours before working properly and confirmed mine was valid. The payment went through the next day without any more trouble. Simple explanation, but it saved me a lot of confusion.

Many thanks for such a thoughtful answer, now everything is much clearer to me.