Well, I would start by completely cleaning off any and all nano lube from the threads since that seems to be the thing that got changed. I'm wondering if a bit of that got onto the area where the spring contacts either the tailcap, or the head, as I believe that is the pathway for sending controls signals for the light. So, be sure and use a clean q-tip and cleaning both ends of the spring, as well as any place where it might contact.
Next, after checking the retaining ring in the tailcap for tightness, I would also the check the retaining ring in the head, as there have been a few reports of that loosening up as well.
Are you using primary batteries? Have you checked the voltage of your batteries? Is there a short, single flash of light when you screw the tailcap back on, or simply nothing?
And last but not least..... did you absolutely double check and make sure that you are putting the battery in the correct way, with the positive towards the LED? Sounds simple, but often that's all it is. Ok, that's the best I can come with at the moment..... let us know if any of this helps.