This can happen sometimes, it is a safety mechanism.
It is very important that you FIRST loosen the nut on the adjuster bar by turning it Counter-Clockwise until it is about an inch above the main pole. Then you can turn the adjuster bar Clock-Wise to loosen the pole from the ceiling and lower the dome.
If you accidentally try to turn the adjuster bar before loosening the nut, it will tighten the pole in place. Once that happens it can be a challenge getting it un-done but not impossible. I personally like to use a good pair of soft grip channel locks or a pipe wrench to get it down when it’s stuck like that.