Several times, I wrote here about autofocus failures of my D600. Now let me share my observation of the significant influence of the position taken on the autofocus. Preferably I choose the backlight and then autofocus works well but lets deviate 20-30 degrees from the specified position and focus slows down substantially. This once again confirms the advantage of backlighting especially in street photography.
This phenomenon has been verified many times and the result is always confirmed. The explanation is simple contrast frame facilitates the focus.