WD-40 has no place in this. WD=water displacer. Use it if your old car's ignition gets wet. Don't use it as a lubricant, because it isn't one, and will gum up after it sits a while.
Possibly a few drops of light machine oil (sewing machine or gun oil) would help. It's hard to know - metal shaft/metal bushing which has corroded? Plastic shaft which has swelled?
Have you removed the knob(s)? On some style switches, it's possible for the knobs to rub if they're located wrong (too low).