We have a tub/shower with Kohler hardware that is probably 20 years old. It is a single handle model with the diverter mounted on the wall near the handle that controls the on/off and temperature of the water. My problem is that the diverter sticks and is very hard to push in or to pull out. If you grab it and push it in and out repeatedly, it will loosen up but after a short while it will stick again. I've tried lubricating it with WD-40 and petroleum jelly but it doesn't seem to last.

Any suggestions on how I might get the diverter to operate smoothly again?

Thanks.