Well let's take a quick look around shall we, I will never pass up an opportunity to go digging around basketball reference. I will try to ignore the seasons from smaller schools before they hit the bigtime and focus on their main time at their main school or schools.
John Wooden, 20th season -- 29-1, National Championship
Coach K, 20th season -- 29-5, Sweet 16
Jim Boeheim, 20th season -- 29-9, lost in National Championship game
Dean Smith, 20th season -- 29-8, lost in National Championship game
Jim Calhoun, 20th season -- 30-4, Elite Eight
Roy Williams, 20th season -- 36-3, Final Four
So as you can plainly see, it is actually quite normal for a coach to experience a lull in......wait a second, the 20s are on the WRONG SIDE!