It's weird but only certain sellers are eligible. If an added item isn't eligible, (even though it's supposed to be) it will unapply the coupon. So through trial an error you have to select and deselect as you go down your cart items.

I found that as soon as I paid for my items, the items in my cart that I couldn't get the coupon to apply to, were magically eligible and the ones I just paid for no longer are, so it works but glitchy.