I bought a box of ten a couple of years ago (this machine has taken longer to finish than I expected!) I have fitted 6, so I have gone through the rest of the box testing each of the other four. I bolted each one in turn to the machine so I could trigger it by turning the ballscrew by hand and connected it to a bench power supply. So, completely independent of the control box, motion controller, etc.

One does not work at all. One turned out to be an "EX" version which only had two wires instead of a three-wire AX. The other two both have this sharp switch-on/switch-off point with a microscopic movement of the ballscrew needed to go from on to off with a flickering stage between. As I understand it, this is not what a proximity switch should do - there should be a difference between on and off trigger points to avoid exactly this situation. I think I have a bad batch of switches.