I have also seen a case where the board was not screwed down into the chassis properly. I realized this when I looked closely at the NUC and saw that the USB, etc. connectors were not properly aligned with the openings in the chassis. After completely removing the board and then reinstalling it back into the chassis, the alignment was correct and the problem with the power switch was resolved. Now, this only applies to the older NUCs that have the mechanical power button on the top of the chassis that, in turn, presses the electrical button on the board. As Leon indicated, the SY NUCs were the last that had the button on the top.
Hope this helps,