That's just it. Those guys were/are juniors and seniors. Underclassmen don't seem to do well/play in it.Why don't you ask this question to Dejon Gomes, Prince Amukamara, Eric Hagg, Larry Asante, Matt O'Hanlon, Ndamukong Suh, Barry Turner, Lavonte David, Alfonzo Dennard, Daimion Stafford? When the defense as a unit gets it and are athletic enough to make plays, it is the best defensive scheme in the country. This year, we don't have the depth in the secondary or the athleticism to make the plays. We also have no depth behind David and Compton at LB. Combine this with not having big bodies at DL to clog up holes and stand their ground, and that is why the D is struggling so much this year.
I know I don't know all the answers, but this is my diagnosis. Once we get better athletes here, we start dominating again.
As Husker Richard said above, you have Cassidy and Thorell starting over possibly more athletic guys because they know the system. This is not getting your "best" players on the field. It is getting your most knowledgeable and that doesn't always translate to wins.
We need a system to get the best playmakers on the field. If it means implementing the system little by little to get 1-2 more wins per year then do it.