I've only used that one time at work, so I cannot really come up with any additional troubleshooting. Have you tried powering both devices off and back on again, the TV and the iPhone? I will have to say someone else may need to try and answer your question if they know more about using the two. I've just seen a couple of different issues in the last couple of days with HDMI issues, and they both had to do with not selecting the correct input.
You might also want to check if there are instructions with the adapter that indicate a specific order to connect and/or power the devices on to ensure connection.