A customer has used a voucher code at checkout and the discount applied was wrong, or wasn't applied, why?
If the discount applied was wrong
First take a look at the voucher code that they used on your voucher codes page (click 'checkout' and then 'voucher codes' from the menu).
Take a look at the voucher code and specifically look at the type of discount provided.
You need to look at the 'discount type' and 'discount given' boxes.
Then check the order to see if this was the discount that was applied. If it wasn't then please contact us.
If the voucher code wasn't applied
For the voucher code in question, check the 4 criteria (indicated below) that govern whether a voucher code will apply.
Now check the customer's basket and see if the voucher code should be applied. For example, have they ordered a product from the appropriate category?
You should also check that the voucher code hasn't been used the maximum number of times, or - if you are limiting customer use - that the customer hasn't already used the voucher code the maximum number of times per customer.