HyperDrive HDMI port is designed to work with HDMI displays and may not work with other adapters or converter cables that convert HDMI to other video ports like DVI.
If the HDMI cable is too long, it might not work as well.
Try connecting using another HDMI cable or HDMI display to make sure that the issue is not with the cable or display.
If you are connecting a 2nd Thunderbolt/USB-C display to the HyperDrive, connect it after the HDMI display, not before.
Restarting the computer, unplug and plug HyperDrive/HDMI may also help to reset the HDMI connection.