Loosey Goosey Control Knobs
Are the pots split shaft or solid? If split, you can prise the two sides apart slightly with a screwdriver (careful you don't go too hard, I've break off before) - this will increase the grip of the knob.
If the issue is that the inside of the knob has worn away, and you don't want to just replace the knobs, you might want to try a turn or two of plumbers PTFE tape on the shaft, that'll increase the grip and width.
Finally, is it the knobs that are loose or are the pots themselves moving in the body of the guitar? If the latter, check that there is a lock ring between the pot casing and the wood of the guitar. And, of course, make sure the pot nuts are tightened fully.
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