It's not enough to just do a job. You have to be able to get the ball yourself. Just have a look at "stoppers" like KCornes, Rawlings, Stenglein, Kirk, Holland, etc and you will notice that they all are able to get their own footy. And in order to do that you have to have sufficient football smarts in the form of reading of the play, awareness and positioning. I'm afraid this is the area Jackson fails in.