They input the code but then click the [Continue Checkout] the cogs spin but the code does not register before heading off to PayPal. So no green tick.
To register the code correctly you can click any where (except continue checkout
Whats not clear to the customer is that it needs to update before [Continue to Checkout]
Either a [Submit] button needs to be added next to the code input box (literally doing nothing but updating the page)or the field needs to be dynamic in updating or a no-discount default in the box to ensure people think and making the submit button Required on this page.
Also a suggestion. The size of the box is quite small on mobile devices and easily missed maybe the text could be in RED or the box size increased so its not so easily missed.
Thanks for reading