grades gone down

I have a few players in mind that lost a little on their grades this year. Im curious about the reasons they have dropped and if they can get their grade back up or if they were just a mistake by John grading them last year.

Josh Butler B- to a c+
Matt Sulentic B to a C
Jermaine Mitchell B- to a C
Darby Myers C+ to a C
Jonathan Herrera B to a C+
Tyler Lumsden B to a C+
Ryan Patterson C+ to a C
Carlos Fernandez-Oliva C+ to a C