This could happen for a several reasons. First, make sure that the card you are using to top-up your account is still valid and that its expiration date is well ahead. Second, check whether your account has sufficient balance to cover the top-up amount.
Please note, a low balance top-up is only triggered once a day, so you may have to wait up to 24 hours to see the money in your account.
If this is not the case and you can see that your card has been charged, please contact our Customer Support. You will need to show proof the money has already come out of your linked account.