What is your MROF set at? If sensi is off (constant red light at back of frame when live) but your MROF is set low that will be the problem. Shove it right up to 25BPS (the maximum with no Sensi) and try again.
Have you got sensi turned on? If it is on (red light blinking at back of frame when live) and you are dry firing that will probably be the problem.