I went back through and checked my achievements; I found that for some reason it didn't show I had completed the final mission on "The Runner". I played it again and it unlocked the sixth story. Thanks.
I had this same problem, the cause of this is that you have to finish all episodes on the same difficulty. So if you finished all but one on normal difficulty and the other one on hard or easy you have to finish that episode on normal to get it unlocked.
