From what you are write I get a feeling that the cause lies somewhere in the connection between the body (tube) and the tailcombo of the Jaxman. Personally I don’t own an E2L but I’m guessing the threads on the tube are bare metal on the head side and are anodised on the tailside. Normally I would suggest to turn the tube 1800degr around, but on my Jaxman the threads in the front differ from those in the back. I’m guessing the fault lies in the threads on the tailside of the tube, they are a tiny bit too short. Try this for checking.
Lay the tailcap of the Jaxman on the table, with the tailspring upwards. Now take a bare metal paperclip and make yourseld a ring the size of the diameter of the tube on the tailside. Lay the ring in the tailcap and then screw on the tube. Careful! Does that work?
If so, you must figure out a way to turn the solution from makeshift into everyday use.

The length of the threads is not always the same between lights and/or between front and back. And sometimes the deviation from the norm in manufacturing is a tiny bit off (threads are a bit too short) and the tube cannot make contact to the tailcombo.