Step 1, remove the movable jaw (there are screws in the middle, usually two screws, one for locking the baseball, one for locking the hemisphere and the other for fixing the other screw).
Step 2: Now you can check whether there is any foreign matter stuck in it, clean the screw to see if there is rust or foreign matter, and add some lubricating oil to make the screw move. 80% of the screws are iron slag or rusted, which is generally the case that the screws are exposed. If the screw is sealed, there will be no problem of the rod getting stuck.
The third step is to check the nut and bearing in front of the fixing screw, that is, the outer ring, and loosen the two screws, sometimes. After the two screws were loosened, the nut became tighter and tighter, and finally the screws got stuck. Add some lubricating oil to the bearing. If you have strong hands-on ability, you can also remove them all for lubrication. Then check whether there is any sign of sliding between the screw and the movable jaw, sometimes it will slip.