Of the 4 ORD machines I tried on a recent early morning arrival, only one was working. No one around to offer assistance. The area had no border patrol. I was contemplating just walking though but knew there was no upside to this. Other passengers continued on to the next section.
I finally got one of the four in the unmanned section to work after I had tried the other three and waited for another, successful, passenger.
When the machines work, it is brilliant.
Ive had the same problem with O'hare. I had to keep moving my passport around before it read it and got me through. I was still one of the first ones through so I did not care.