My 5D MkIII can not archive focus on subject that has solid color with light distributed evenly on the surface. example the walls, i tried this on all my walls with different colors, green, coffee, white, the test i did was during the day, with enough light.
my pink cabinet, powered off monitor screen, etc.
I must move the focus point to some place that have color difference like edges in order to achieve focus.
my 60D is more successfully achieving focus on these situations on the monitor screen and pink cabinet.
is it the same with your camera?