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While it may be the exact fix for this specific problem, have them think of "fretting forte" at no matter what volume they are playing. They are probably not used to feeling frets in their left hand and need to be more aggressive in their fretting (I have been told to think of pushing both down into the fretboard as well as into the fret, the latter of which they are not used to.)

Marty "not an ebass virtuoso"

A bass player needs to be an animal. The best bass guitarist I ever worked with, Orville Henry Harrison (Big "O"), used to pull the strings as high as his chin. He would break steel "A" strings. The percussive effect, even at low volume was enough to jiggle your hair.