I agree with comments about voltage - when I was setting up my machine I tested the proximity switches on a variable-voltage power supply and they were getting a bit flaky once down to about 9V. 12V gives solid performance. I run mine on 24V, partly to allow for any voltage drops where I have a pair wired in series (where an axis has + and - limits, I pair these together to share an input). In testing, I wired 4 in series, just to see what happened, and that worked OK as well. Not surprising if you look at how they work, but it's comforting to know that it's also true in practice as well as in theory!