Try changing sources on the TV away from the Roku then back to the Roku.
My one Fire TV box does this, I turn on the TV no signal, change sources then change back and boom it is suddenly there.
I think sometimes just turning on the TV does not trigger the HDMI port correctly. I am not sure, I only know doing that fixed the problem. It is a George Jetson problem, I have to hit a button twice now.
