Running up the middle

I am opening this thread just to focus on this aspect of our defense. Good teams keep attacking us in the middle. We are good against stretch plays but we are losing consistently when teams keep pounding the middle over and over again. I thought that is what our philosophy is but teams who can do that will keep drives alive, eat up click, tire the defense and if we over commit they go over the air. So here is the question

1. Is it scheme
2. Are players hurt
3. Is it size issue where we don't have enough beef.
4. Are our players playing hurt.
5. Are we too quick that we can't hold ground.
6. Are we pass rush focused that makes us vulnerable? We were much stout 2 years ago.
7. Is it psychological when we come into play that we already have this and lose the battle not playing with the same intensity
8. Are we substituting too much in the game that affects continuity of personnel.
9. Is our substitution in the line make it predictable what we plan to do for the opposing team adjust.

10. Our offense is too fixated in explosive drives and time of possession keeps defense on the field longer.

lets keep this thread to discussing defending against run defense as the next game we will be challenged with the same issue.