I have the same problem with all of my cameras loosing the IR function. I noticed that all of the cameras have the IR leds on. So knowing a little bit about how the camera works, I did this to test my theory.
During the day, the cameras are for the most part in color and fairly clear.
At night when the cameras work, I have some fairly clear video. Lately though, the cameras have been acting up and not switching from day vision to night vision.
Here is a simple way to validate that your camera is switching between day and night vision. I will say that this method has its drawbacks. It must be done at night, you will need a small flashlight, you need fairly good hearing and vision, and you will have to have a smartphone with the appropriate application installed on it.
Using your smartphone with the appropriate app installed, I use Zsight on an iPhone 5, bring up your app to watch your cameras before, during AND after shining a small flashlight at the camera under test. You should hear a clicking sound from the camera as well as see the IR leds turn on and off when the light hits the camera.
I did the test on all four of my cameras and they are working just like they are supposed to. However, I have a sinking feeling that I may have to do this again in the future.
I have done voltage tests on my cat5e cables I built myself and there is very little voltage loss. The furthest length of cable is 55 feet while the shortest cable is 25 feet.
I hope this helps. I am coming to a conclusion that the photo-detector is sensing the IR leds and keeping the cameras switched in day mode.
Finally, an interesting note to mention; the flashlight I used to conduct my test was a "Blacklight Master" with 28 UltraViolet leds. I am pretty certain that an IR flashlight would have the same results. Yes, a regular flashlight works well also.