BLF Special Edition A6 XPL LED 18650 Flashlight

I am a novice flash-a-holic and am having trouble geting my BLF A6 light working. I have another BLF Special Edition A6 that is new, but the original light is one I used every day.
All of a sudden it does not light. I removed the old tailcap and screwed it on the new light and the old tailcap works on the new light. I cleaned the inside of the old tailcap and checked the 18650 battery. Both are OK.
If I remove the old tailcap and use a metal tool from the side of the case to the battery, the light works.
If I put the old tailcap back on the old light, it doesn’t light.
If the light works with the “tool process” used on the old light and also works when I put the old tailcap on the new light, I would assume the old tailcap is OK and the LED end of the old light is OK.
I don’t know what to check next and how I can fix it.
I would appreciate any help diagnosing and repairing it.
Thanks.

Ted,
Have a look at the retaining ring inside the faulty tail cap. I recall some lights having a similar problem when this flashlight was first introduced.
Use a pair of needle nose pliers or tweezers to engage the holes of the retaining ring and then tighten (snug) into place. The ring might be preventing contact from the cap to the body, because it is above the non-anodized area of the tailcap.

Paul,
I found the answer from Flashy Mike of BLF Forums. The center tube has different thread lengths. I had the shorter thread length connecting the tailcap so it wasn’t making contact. I reversed the tube so the longer thread length was screwed into the tailcap and now it works perfectly. Thanks again for your input.