The truth is, everyone is mediocre at getting REDcards. There are a few cashiers who are able to "champion" REDcards and get numbers no one else can (invariably, there will be one or two of these at your store -- we called ours the REDcard Queen), but these cashiers are either naturally good at selling to people (most cashiers are *not* natural born salespeople), have a few tricks up their sleeve and/or are very good at making guests who don't sign up feel a little bad about it. Our REDcard queen was eventually fired because it was found out that she was being incredibly deceptive when it came to getting guests to sign up, sometimes outright lying about the benefits and telling people it was just a rewards card. She used typical salesman tricks to make REDcards look like the greatest things since sliced bread. Can't say Target does a whole lot to deter this since the damn cards matter so much to a cashier's job.
Look, I became GSA and my REDcard numbers as a cashier were not great. They aren't everything. The cashiers getting all the REDcards usually stay where they are, because why would any store want to risk losing those numbers by promoting them or moving them to other depts?