Guidostrunk
Headphoneus Supremus
There may be a clip inside the knob , that may have compressed to much when being built. It looks like a pressure fit knob. Best solution is to glue it on or see if you can bend the clip inside the knob , outward. Hope it all works out for you. Cheers.
It would be easier for me to find a thin metal object. XD
Thanks for your help.