Not sure why. The Other functions, HDMI time code setting flashes whether set to on or off but there is no signal to the monitor. Monitor works fine on my EX1r and the camera HDMI works to the TV monitor. The Marshall monitor is a 1280 X 800.

I am brand new to the C100 but I'd have to wonder if you have your camera set to 24p instead of 24psf. I know some monitors cannot handle a true 24P signal where as 24psf is embedded in a 60i stream for better compatibility. I do not know for sure how this works yet as I have just had my camera for a few days, but when I had the Black Magic cinema camera several HDMI monitors would not recognize the true 24P signal coming from the camera.