Firstly, don't forget to read the Ts&Cs for the specific voucher you are using.
When you are applying your voucher, you'll see a green notification bar at the top of the page. If this notification pops up, your voucher has definitely applied.
You'll see what the voucher is worth and how many boxes it's for. It may not look like the price of your first box has changed but we can assure you, it's applied.
You will also be able to see your discount code in the 'Gifts and discount codes' in your account area when you've completed your sign up, so you'll only be charged the discounted amount.