Customer Frank received a lid knob from us and it would not screw down all the way. We have had the occasional report of defective threads on the nut inserts, so we sent another. That one had the same problem. But then Frank had an idea.
After I wrote this morning, I had the inspiration to shoot some WD-40 on the threads of the lid and into the threads of the knob. I unscrewed the knob, put the lubricant on, and tried again — and it went further. I did that several more times and each time it got closer to the bottom of the threaded post… and finally got the new knob to go all the way down to the surface of the lid.
Corrosion on lid knob screws is common, as water gets trapped under the lid knob and around the screw, and the dissimilar metals used between the screw (stainless steel) and the factory nut insert (sometimes aluminum) can promote corrosion. That happened to be the case for Frank and a little WD-40 helped the knob go down all the way.