what actually works to get a zipper back on track?

if one side has come out of the slider: open it fully, prise the little metal stop off the bottom with pliers, feed both sides back into the slider, then crimp a new stop on, or sew a few tight stitches across the bottom to act as one. if the teeth are just splitting open behind the slider, the slider itself has worn open. squeeze its back end gently with pliers, both sides, a tiny bit at a time and test. that second one fixes most jeans and jackets.