Howard,
I can't tell from the photo but on some of these jacks there was a small metal plate with a threaded hole which slid into a slot on the insert and served as a nut for the set screw. Check your bass carefully to be sure there are no other loose bits rattling around. If you find this piece you should be able to reassemble it all. Another tip is to pull the entire jack out of the bass just far enough to get a straight shot at that adjustment screw...
If you are a solderer, instead of repairing this piece I would take the opportunity to replace the entire jack with a new one. Just make a map of which wires go to which pins!
Jimmy J