The problem is that it dosnīt seems as these round black knobs are avalible, so be very careful.
They are fitted with a little tiny screw, and if you carefully loose these, take the knob out and use a plier to the metal part in order to be more firm if you want to lubricate and "push & pull" untill it goes little easier. That fasten the knob again. But DONīT
overtight the screw, or you will break it as it is old, not flexible and very frigile.
I broke one of mine while tighten it just too much and have looked just everywhere for replacemnts without any succes.