Don't expect Nebraska to sign a Top 10 class very often. Geography alone makes that extremely difficult. Recruits from Nebraska and bordering states are not going to be as heavily scouted or highly ranked, compared to their peers in California, Texas, and Florida. Northerners also do not have year-round football, nor do they face as high of competition. Lincoln and Omaha are also not major TV markets, so there isn't going to be as much hype about a recruit coming from, or going to, Nebraska.
Even when Nebraska was winning championships, it wasn't as sexy of a destination as some other schools. Recruits from major metro areas still think Nebraska plays ball in a cornfield, and students ride cows to class. Which is weird, because Tuscaloosa, Alabama is quite a bit smaller than Lincoln.
With that said, recruiting a Top 15 class every year, plus the walk-on program, plus excellent coaching can still make Nebraska a contender. The trick is to do that consistently, and have some coaching stability, so the players can actually thrive.