I suspect that the grooves in the sides of the pins are too small so the string is held by fricion rather than the ball end. After a bit of vibration - twang. Just like a bow and arrow really!

You could try carefully enlarging those grooves with a sharp object.

Or you can get ebony pins with no grooves quite cheap (about GBP 10 over here)and practice on them.

I prefer bone pins for brightness and sustain but several friends who had quiet or muffled guitars have found brass ones useful and they look good too.