Sounds it is a common issue eg
https://www.cloudynights.com/topic/600195-vixen-porta-ii-slow-mo-knob-fix-for-some/
If the shaft is the same diameter, swap them out for something like these perhaps?
EDIT: from the one review on there it seems it is indeed a good solution for the porta mount
https://www.rothervalleyoptics.co.uk/skywatcher-slow-motion-cable-set.htm
(other retailers are available)
Cheers
Robin