What Is Going On With Runs Up the Middle?

Mentioned this briefly on a post on the Red Board, but wanted to ask if anyone saw what is going on with giving up big runs up the middle.  Last week Steven Jackson torched us up the middle with a nine yard average, most of it in between the tackles.  This week, Morris hit some big ones up the middle.  Are we just set up wrong/stunting for example, or are we just blocked off the ball.  This includes Gerald McCoy, who plays DT.  Especially as you all called out the run defense, was wondering if Scott and Mark especially might have some extra insight on the X's and O's.