I had a couple of the gray plastic knobs pull off while opening stuck windows. The set screws were not tight enough. I tried new set screws. You can buy the set screws at Home Depot or Lowes. Take a knob with you to size the threading properly... be sure to buy a hex wrench that fits the set screw (or 2), as they wiggle loose eventually.
I then tried a different approach - I removed the metal push rods going out to the windows (after you remove the 2 window screw attachments, the rod just twists and goes out the opening on the side of the window). Then, I used a tap & die set to cut threads the round end of the rod. I held the rod steady in a vice grip, for threading, using cutting oil. Then I sanded and painted the rods, re-installed them on the window, and then screwed on new small round pewter (handles) for the window knob. It's really sturdy, and cannot pull off, and looks pretty good. I actually used "newels (sp?) that hold lamp shades onto lamps (about $3 each) for the knobs.
If you try this, do not thread down too far on the rod, as it might interfere with closing the window all the way, if the knob is threaded too far down on the rod, it can bump against the window frame when the window is closed, due to the angle of the rod.
The FunderStream rehab is complete, and we are hitting the highway!
1998 Excella 28'
2008 Toyota Tundra 5.7L with HAHA