1,The cable may be damaged,pls change a cable to test it.
2,Power supply may has problem ,pls replace a new power supply for it.
3,Pls make sure DVR and camers are in the same pattern.(AHD/TVI/Analog).
4,Pla make sure the format of DVR is the same as camera's:Pls go to your DVR’s Main menu--Set up--Basic--Video Format (Note: Choose the same signal system as camera ).