This can occur for several reasons so let's start by checking on the physical ones. First make sure that the camera is connected and that it can see the whole screen. In some cases the camera might need to see the entire projected area space as well. Use ScalableDesktop camera panel to help you with aiming and connecting the camera.
Next take a look at the image being shown to you in the software and see if there is a ceiling light that is in the camera shot and close to the screen. Sometimes lights in this position confuse the software because they are brighter than the projected image. Turn off the lights if this is the case.