If the linebacker can't get to the gap because he has an offensive linemen on top of him, it makes it kind of tough to stop the run. In some schemes, the linebackers are sent to attack the gap immediately, which allows them to beat the lineman to the punch. Nolan plays a more read-and-read scheme where the linebackers have to hold up for a second before attacking. So in order to feel the gap they have to outmaneuver or outmuscle a lineman who outweighs them by 50-plus-pound or the DTs have to hold up the OLs at least long enough for the linebackers to have time to beat them to the gap. Our DTs don't do that. Plus, they often get pushed out of their gaps, so if the linebackers are flowing to their gaps, there are holes for the backs to run through.
Although all defenses need a strong interior, Nolan's especially needs ones who can cave/congest that middle and allow the linebackers to be free to run to the ball. Good assignment football only works against the run if the DTs can handle their assignments. Ours cannot. Their assignment is to either tie up blockers or win their one-on-ones and disrupt the play, and we've seen only a few times with either of those have been accomplished.