Had the same issue with mine. Used Tamiya plastic cement (the green cap) to adhere the knob to the machine and the plastic welded on itself. Not had an issue in 14 months. Only works on ABS plastic though.
how do you know what type of plastic its made out of? is there a marking on the knob?
is the loctite plastic bonder the same as two part epoxy? or is it different? the bottles look the same at the store.