How can I decrypt data on a replicated MSSQL database?
I have imported the certificate and the key and I have created a symmetric key however, whenever I try to decrypt the credit cards, it just returns NULL.


If the symmetric key you are talking about is one you've manually created in the user database then you'll need to create it with the same initialization vector as the previous one by specifying a Key_Source and the same algorithm. If you haven't done this (or restored the database containing the same key) then that's your issue.

Here is a walk through to show you how to create the same symmetric keys in multiple databases.