For some reasons, several senate incumbents who won reelection underperformed their 2012 numbers (excluding Manchin and Feinstein for other reasons): Klobuchar, Gillibrand, Cruz (strong challange), Barrosso, Bernard, Menendez, Whitehouse and Stabbenow.
I made a thread about this a few months ago, and I believe that polarization is the major reason why these Senators underperformed. It may also help to explain why certain other Senators (i.e. Hirono, Cardin, Kaine) did better this year than in 2012. Hirono and Cardin in particular, registered sharp increases (of ~9 percentage points each), over how they did then. Hirono went up from 62% to 71%, and Cardin from 56% to 65%.
