Following are the reasons for your reference：

1.Video signal is too weak due to the loose of connection between Video input cable and the video input connector on device back mainboard，
please check if the connection is fastened.

2.The format of video input and output is not matched.please move the mouse to the bottom of the toolbar and click the “video signal”button, 
set the correct “CVBS output format”,generally, PAL is in China

3.Video signal attenuation due to transmission distance or cable damage