Are you inputting a video signal into the HDMI IN connector that exceeds 1080p resolution (such as 4K video)?
The VC-1-DMX does not support video resolution input greater than 1080p. Check the format of your video input signal, and lower the resolution for input.
If a copy-protected (HDCP) video is inputted to the VC-1-DMX , the receiving device connected to the HDMI THRU connector may need to support HDCP as well.
Check the specifications of the receiving device connected to the HDMI THRU connector.