The discount hasn't been activated properly (either you forgot to activate it or there's been a cookie issue). In that case please contact our Support Team directly via firstname.lastname@example.org. They'll be happy to look into it.
Please note: no discount can be applied if you subscribe via iTunes or GooglePlay. We don't manage these payments and can only offer discounts for purchases made via our website - that's why we never link to app stores in our emails.