Open Beam with Arduino Sheild and Limit switches

Hi Folks.
I know a bit about electronics and have been hacking the Marlin code of my 3D printer for a while. I also use Arduino's in various projects.

Ok. I am making my CNC machine and am using an Arduino UNO with a Arduino CNC Shield Version 3.0. I am also using GRBL 1.1

I can get the steppers to move correctly but I am having a nightmare with the limit switches. I have mounted the NO on the CNC and checked them with a multi-meter. I have plugged the switches in to the corresponding + and - pins (Yes I did read the instructions and swapped the Z+ around)

But nothing I can do seems to make my CNC recognize the limit switches. I have looked over the web for several hours and cannot seem to find a solution. If necessary I will mod my GRBL code but would prefer to do that with some advice/input for others.

Many thanks
Dave