You can check that the output signal is being sent appropriately. Go to Widgets->Mavlink Inspector->Vehicle 1->SERVO_OUTPUT_RAW
When you push the buttons for lights, do you see channel 7 change? If so, then it is an issue with the hardware/wiring.
This sounds like the camera cable is getting pulled loose and the camera disconnected from the computer inside the ROV. Can you make sure the cable has enough slack at the extremes?