Might be a burr or piece of rust in the nut thats hanging up on the wire. At any rate, do not twist the tube. I would probably work the nut up the wire until I could see the inside from the back and free it up. Its got to turn freely to tighten up in the block fitting. Its there to trap the bulb against the inside of the fitting that screws into the block with out twisting the tube.
Possibly, by moving the nut all the way down against the bulb, you can work the wire spring back out of the nut enough to tighten it. I guess it goes without saying, be careful there.