Should I wire my CNC limit switches as NC or NO?
Your switches are probably working against the pull-up resistors in the controller. These are probably 50 kΩ - 500 kΩ. When your switches are closed (as they normally are) there may not be enough &quot;wetting&quot; current to break through the oxidation of the contacts.
I recommend that you try addition of a lowish-valued pull-up resistor to positive supply to ensure that a few mA flows through the closed contacts. 1 kΩ would do the trick. If you want you could lower it further and add a series LED.
A squirt of contact cleaner may help if you can get it in there.