It could be a defective switch although if it is showing as active in the signals test, then it would be an intermittent fault - which would be unusual in a solid state hall effect unit. If you have a magnet handy, move the gantry away from the switch, set the machine for signals test and move a magnet close to the switch.

Other things to check :

1) All cabling. Check that the signal plugs into the controller are good.
2) Reference Speeds. Assuming the switch is good, what are they set at? Does reducing the homing speed make a difference?
3) The magnet has become degaused. Unusual but it can happen if they are knocked or for some reason heated up.