I have been having the same issues with CC2014. I had made a topic before but did not find a solution.
"The backdrop settings are preventing the preview from showing brush changes."
CS5 would allow me to select either a layer on top or bottom and alter.
CC2014 no longer allows me to alter the bottom layer with mode: in front, using: any layer on top
I use this function to preview changes between layers. Been doing it for years.
I have already updated drivers, and am familiar with show backdrop and ... to keep it short, it worked before in CC2014 it does not work.
NVidia Quadra 600
Win 7 64