When my full kit arrived it contained limit sensors for home/end for all the axes - i.e. 6 sensors. Although I soldered them all, lazily I only attached the three home sensors to the build.
During testing with the stepper exerciser, I started getting some unusual symptoms. sometimes the stepper would only move in one direction (e.g forward and not back) and the 'home' buttons did nothing. Trying to find the range just sent the motors heading away from the sensor.
Finally, I worked it out and tested the sensors separately. I'd wired the 3-pin plugs backwards (+5v/grnd reversed) and this had blown the sensors. Even when there was nothing there, they reported closed. I guess I'd blown the IR source.
Luckily, since I'd been lazy earlier, I had three untouched boards. Once I'd rewired the 3-pin plugs properly, the arduino test program correctly reported open and closed when I blocked it with some card.
I replaced all three sensors and the homing function worked properly. The clear acrylic from the BfB kit doesn't block IR, so I glued some card squares to the moving pins to block the IR.