after a brownout my some of my cameras becomes black screen. resetting the power supply turns them back on. how can i prevent this? what is causing this?

what DVR is it on?
What Cameras and how are they powered?

it is AVC760. i use a 12V 6A regulated power supply to supply power to all the cameras. the cameras i used are outdoor bullte IR camera

Is it a Distributed power supply, or a single plug in transformer?
Maybe the cameras power board doesnt reset or something.
Do you also have to reset the DVR or just the cameras?

it is a distributed power supply. just turn off the power supply and it will be up an running again. it shows black screen after the brownout. don't need to turn off the dvr. so it is also possible that the camera board did not reset?