Lower Z-axis nut damages

Up to now very happy with my nano.
Building it was very nice, really enjoyed it. Then from the first print on it went fine, great prints. Now I haven’t used it for some time I am running into some problems.
The lower M4 nut that keeps the Y-axis holder in place is damaged. The thread in this M4 nut was completely worn off. Then the y-axis holder just drops down. The M4 nut can just slide up and down the threaded rod. The thread on the threaded rod seems still to be okay.
I replace the original nut with another M4 from the local hardware store but a couple of day later same thing again. Again the thread in the M4 nut is completely damaged.
Any ideas?

I had a similar issue… Endes up replacing the nut, coupler and rod. I also added a bit of oil to the rod the not turns easily and don’t get stuck. If the nut gets stuck, then the servo will wear out the nut very fast.

Thanks for the reply. Seems that that would be case, If the nut is stuck and the rod keeps turning. The nut will suffer.
I will give this a try,