I have 15 of the Dahua 2431's running in Blue Iris and it's been working great for over a year. This week two of them started showing no signal in the Blue Iris console. I can ping their IP address, and I can access the WebGUI, but the video preview is just a black screen. I've tried rebooting the cameras and the trouble persists. Have these two cameras failed in the same week? Or is there something else I can try?