Can I have it if you decide to 86 it?
If there is no movement, I'd say the problem is in the light fixture itself. My guess is that the problem would be easily apparent if you just took it out and looked at it piece by piece. That's something I've never done so I couldn't make any guesses as to which particular piece might be causing the problem, but it's probably something simple, like maybe a bug got into it and is causing limited movement of one of the pieces. (That may sound far fetched, but it actually happened on one of my motorcycles one time. A bug just happened to be crawling across the contact of my starter switch one time when I tried to start it, and the resulting "pieces" that remained prevented the switch from making good electrical contact until I took it apart and cleaned off the contacts. Once it was cleaned, no more problems.)