Npcorc

Hi, can anyone help. Im an electrician and dont normally touch cctv. My customer has a CnM secure system with 4 cameras, one has stopped working, i have now bought 2 different cameras that look identical to the ones that are working and neither come up with any picture when connected. I have connected to an existing supply that is working to check its not the cable or channel, but cannot get a pic with new camera. Am i missing something???