First of all, check if it's not a top-up with code. If the top-up requires a PIN code (mentioned on the product at the time of purchase), verify that the mobile to top up has entered the correct PIN code by calling the correct number.
If the recharge is without a PIN code (the majority of cases), the transfer is almost instantaneous. It can take up to 10 minutes with some operators before the top-up is credited to the phone.
If despite this you still have a problem, please contact us at firstname.lastname@example.org or via the online form.