If you start a inprivate browsing session or equivalent the voucher will appear. Also if you delete all you temp internet files, cache and cookies the voucher will appear. In the link I posted before it states "So you have another page on the same shop processing a payment or you tried ti cancel a payment by clicking the previous page button of your browser instead of clicking on "Cancel and go back to the merchant's website" link." which I think is correct.
Then on further investigation I noticed that when the voucher is not available the browser you are using at the time has a block with a paypal cookie ( or whichever payment processing you are using) . I think this is then allowing the code
$('.qty-field.cart_quantity_input, .cart_total_bar, .cart_quantity_delete, #cart_voucher *').remove();
to be activated.