Peculiar z-axis motor issue

I am generally able to home my machine and move the spindle on all three axes manually in both directions (up and down, back and forth). However, when I move manually to a particular XY location (say, x=100, y=80) and then try to lower the spindle in the z-direction the motor vibrates but does not move. I was thinking about trying to fix the problem by adjusting the pot that controls the voltage to the z-axis motor, but can’t imagine why this problem would occur specifically in this one section of the coordinate system. I appreciate any insights others might have on this. Thanks.

Check your wiring. Just had the same problem appear out of nowhere. Turns out a wire was loose in the connector at the controller/power supply.

I’ve had issues like this that only show up intermittently. Turns out that the wiring was too tight going into the drag chain and it was flexing at the crimp connection, which was slowly pulling the wire out. It took a long time to figure it out because I kept jogging the gantry to get access to the wires, finding no faults, then having weird motor issues when running a job.

Thanks for the suggestions. I checked the wiring and made sure all the moving parts are able to move smoothly. Nothing obvious there. Based on the sound it was making, I then adjusted the pot on the shield to allow greater power. That seems to have worked. Still puzzled as to why the problem was occurring in one specific region. Must be that greater stress is being put on the motor there for some reason. I have to remind myself that this type of mystery is part of what makes the whole design and build process fun.