They used to work fine previously. It has always been connected to the same device (Samsung UN32M4500A). I have tried replacing the HDMI cables and nothing shows on video screen. The HDMI cables are 3 feet long. These cables work fine if I take the coupler out. The video screen also works fine without the HDMI cable connected and videos can play independently through FireTV or with HDMI cable plugged if I remove the coupler.
how can I fix this problem?
According to your description, this should be your HDMI coupler problem. It is recommended that you change to a new HDMI coupler!