The inner brass tube of the counterweight becomes detached as the glue dries out.
You can remove the counterweight and unscrew the inner brass tube.Take care to note the position of the tube relative to the outer weight though,so as to have the correct weight calibration when you reassemble.
Once exposed,you can reglue the inner tube to the rubber bung on the arm,using superglue.
Reassemble and recalibrate tracking weight as necessary.
Nick