With a Canon T3i mounted in a camera/flash tower, the camera cannot focus in low ambient light with dark complected subjects. All the settings are preset as this is being used in a drivers license office in a booth. It works fine otherwise. I swapped out the camera tower with another (different Canon T3i, different flash tower) and had no problem in same dim light, same dark subject. Why does one camera have this issue and not the other? Same make, model, lens, settings.
One possibility to check - is the AF assist light turned off or failed on the problem camera? You say the settings are the same - have you checked all of them, including ones hidden in the setup menus (which is where the AF light on/off one is usually found) and the selected AF point/mode?
Some of the menu settings (at least on my Nikon, but I suspect Canon is the same) don't show anything in the viewfinder or top panel LCD, so it can be easy to miss changes unless you go through all the menus :)
The relevant menu on a Canon 70D, yours should be close: