Just to be sure make sure your clutch handle is pulled in and locked in order to get the flower nut to turn easy , and lock screw removed. you probably know this but just making sure. Then I guess put a drop of WD 40 on the flower nut threads if corrosion exists?
If this as been done something is wrong. Give it some love taps if all else fails. Works on occasion. I've never had one stuck, normally very loose once the clutch is pulled in. I've worked on at least 10 different engines, and Crassus probably in the hundreds. Let us know what you figure out.
Ps. That Chinese factory grease can be more like glue when old.