Buy Now Button direct to Cart Discount codes not working...

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I've added a Buy Now Button on one of my website pages and it seems to work fine...with one exception. I'm trying to test out two of the discount codes I've setup in Shopify, and when I enter the codes on the cart that pops up when the Buy Now is clicked I get the following message.

XXXXXXXXXXX discount code isn't valid for the items in your cart.

How do I make all discount codes valid for these carts?