You can only get codes from wrecks which have not been looted before. The wrecks will have been harvested by someone else, but after it's been looted once, it can't be again till next server restart.
I still think that codenames are... not Uber but worth spending 20m each on them (that price is exaggerated). I love codes, and buying and harvesting them is fun, thrilling and adds more awesomeness to disco. Hope more codes will be added in in future.
