max and min along with z axis problem

my machine is built and everything works fine except for one problem. when I move the z up approximately 2 inches the z motor turns but the z screw doesn’t the coupler that connects the z screw to the motor also turns. this happens the same with the z in the negative direction. I’ve ran many tests with myself and two others watching the machine and we cant find anywhere where the machine is binding or seems to be hitting its limit. thank you for your time. final note the motor and coupler turn but the screw doesn’t. The coupler turns while it is clamped onto the screw essentially destroying the coupler because the threads of the screw destroy the wall.

How does it turn by hand? Should be very easy to do with one finger if you use the knob.