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.
The real problem on the La Specialista is the grinder burr gets all clogged up. The knobs are not a big deal since they can be fixed in five minutes. The grinder is a nightmare to disassemble. The seals are proprietary. Had mine apart three times in two years. If you're thinking …
and this is why i said to buy a real machine. Separate grinder so you dont have to deal with this nonsense.