Different teams have different issues:
USC excelled while there were no NFL teams in town and they were the big show. If you couldn't be seen courtside at a Lakers game, you wanted to be seen at a Trojans game. They reaped the resulting money and exposure to recruits.
Miami similarly excelled from 1983 to 2003. Dolphins generally won a wild card game and then lost while the Hurricanes were winning national championships. Before that, Dolphins were making deep runs and going to the superbowl. Though the Dolphins got worse, Hurricanes never recovered after Coker, and Bobby Bowden and Steve Spurrier became huge problems for them that they have never recovered from. I though Richt was going to be the answer, but it seemed he was more interested in relaxing in Miami than winning.
Nebraska's problems were already covered above.
FSU is a good coach away from returning to prominence.
Michigan's biggest problem is the population leaving the midwest and moving to the south. But they're on the right track and I think will become a consistent 2nd to Ohio State like LSU used to be to bama.
Tennessee just hasn't recruited as well as Florida and Georgia and they can't quite be as selective in their coaching searches. SEC East is a lesson in good coaching. Fulmer, Spurrier. Urban, and Smart are elite coaches with elite reults. TN will have a few good years, but generally stay 3rd in the east unless they can find a good, young coach and give him enough time. Very difficult to do.
t.u. however, is by far the biggest embarrassment in the history of ALL sports as far as I'm concerned. They spend more money than anyone, they're in the largest city without real professional sports, they are (along with us) in the absolute best physical location in terms of proximity to elite recruits, and they have the history and facilities and fan support to dominate every year. They have their own tv channel! If they could get even one above-average coach, they could win a lot. They can win conference titles with an average coach, and should be able to win it all with just a good coach (not even great or elite). How they've been this bad for a decade+ is inexcusable and embarrassing (and I'm loving every minute of it!).