Lightning to HDMI dongle

Why doesn't my iPad screen show up on my Samsung TV when I am connected via an Apple Lightning to HDMI dongle. The HDMI icon does not show up for the HDMI connection I am using but another connection does show for another accessory. I have switched the dongle to that (presumably working) HDMI cable and the iPad connection does not show as an option. The other accessory now shows up on the original (iPad) HDMI connection.