Cd Keys

i have many cd keys for cod-2 but only have got a few to work and they dont always work all the time, most of the time when i try to enter a server it will say bad cd code, but sometimes it will let me in.The servers that let me in instantly r the ones where i have to download a mod or something, the other cd keys i have say cd key in use, im guessing those cd keys r being used.Anyone no why a server tells me a cd is bad and sometimes it works.