Only the biblical of Luther's views on Mary were accepted, not the over the top Catholic influence. There was no scriptural warrant for those views so they didn't make the cut to the Book of Concord.
You might want to read this....
The Immaculate Conception from catholic encyclopedia online:(http://www.catholic.org/encyclopedia/view.php?id=6056)
No direct or categorical and stringent proof of the dogma can be brought forward from Scripture.
To date no catholic can refute this.