This sitar kunti has been broken. Since it is an old kunti, the ball is a little bit smaller than recently made new samples.
So, either you change the whole set, or you repair the broken specimen.
Cut the ball from the pen.
Drill a hole in the ball.
Make this hole square.
Find a new pen, make one end equally square.
Make it fit properly in the ball’s square hole.
Apply glue and let dry.
Finish according to original.
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This is not the point where the KUNTI fails. It is the stem or THE PEN as called. The Pen gets worn or smoothed out with use , consequently looses its grip.