Abe,
Should all fail (the track itself may be worn) and a replacement is not readily available, there is one last way out. One can mechanically move the slider to an adjacent unused portion of the track, which is usually quite wider than required. This will however require precision and an ability to do tiny jobs, preferably using a magnifying glass and sufficient light. You will have to remove the pot(s) and disassemble, making notes in order to reverse the procedure. Normally the slider has two 'prongs' sliding on the track, having worn a circular 'path' with time. With a tweezer or similar (best having removed the spindle proper) one might be able to position the sliders to move on a virgin part of the track between or either side of the worn paths. One needs to keep the contact part of the prongs as well as the track pressure similar than before.
Repeating, it needs a steady hand and preferably experience, but can save a control when there is no other way out. I have fortunately been able to perform this several times and with (so far!) no complaints. Again, before replacing, turn the spindle back and forth several dozen times.
Good luck!
