Do you mean that you are entering the code and it is not letting you download them as the codes have already been used? In which case, did you buy a brand new copy or a used one? If it's a used one you almost certainly won't be able to download them as the code will almost certainly have been used by the first owner?
If you mean that you have downloaded them and are entering the code again, you don't need to do that - they'll show up in your purchase history. In that case, does the region of your disc (North America, Europe, Japan, somewhere else) match the region of the PSN Store you downloaded the packs from? Did they install?