I do not know what kind of knobs come stock on those basses, but you have two options.
If there is a small screw in the side of the knob, called a set screw, it's just a matter of tightening it. Usually a small flathead will do the trick.
If it's the kind that simply pushes on, and is supposed to "grab" the shaft, all you have to do is widen the part it grabs on to. Mostly likely the shaft of the pot is a two-prong jobber, kind of like an electric plug. Using some pliers or other tool, simply seperate the two prongs more so they are every so slightly further apart. Then the knob should grip just fine. Be careful when doing this- not a lot of force and only a little bit more seperation should do the job.
If neither of these seem to solve the problem, compare the knob with one that does work- it may be that the inside of the knob may be stripped, in which case a new knob might be in order. Good luck.
