I am unable to many channels in full screen to fire stick HD. Seems to be all non HD channels (premiums)
non-HD channels normally will not fill the TV's 16:9 ratio screen as the SD format is 4:3 . non-HD progrmas will have curtains on both left and right. HD programs may appear as a postage stamp with curtains all around the screen. A set top box wold assist in expanding the screen to fill it although it may be distorted (for example in zoom mode the top and bottom of the actual video may be cut off).