D800 can't change aperture in aperture priority

Just check if you set ISO to AUTO. If "yes", change ISO to manual setting and see what happens. I experienced the similar "problem" until founded that when I shoot in A Mode, sometime I had difficulties to change aperture. Instead of changing aperture camera changing ISO. I think we have to look into it deeper.
Kindest regards from Down Under
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Nicholas Sherman, ACS
web: http://www.nsherman.info