yea ex-vin's got it right. in order to playblast from the camera you want, you've first got to have one of your viewports displaying what your camera sees. (like he said, under panels, go under perspective and then choose the camera you wish to playblast.) a playblast only shows what the active viewport shows... so make sure that you've got the camera you want to playblast assigned to one of your views, and then make sure that viewport is active.
Accept no substitutions.