When a student just won't dig in, I start doing a lot of encouraging. Have the student pick something easy, that they know cold. It could even be a roll, or at the most basic, a 1/5 pinch. As they play it continuously, tell them to keep playing louder. If they are especially weak with one finger (index) tell them to play that finger ESPECIALLY loud.
At this method's most basic application, the student should play just one string as loud and clear as possible. Just an open note and make sure it sounds strong and has bite. Keep encouraging. My bottom line is to tell the person "If you can play it quite a bit louder on this next hit, imagine (vividly!) that you will receive one thousand dollars. Imagine it vividly, and now pick. Sometimes it will be amazing how loud they can pick (time-tested technique at banjo camps)!
Then say, Let's see if you can do it again.... And again.... And again-- but without having to strain to do it. A good big attack on that string, feeling relaxed and strong..
Then, the pinch, then a roll. Always has to stay strong. Then a TITM roll with a slide on the 3rd string. After a while, a habit will form of making sure that every note comes through. Once a player knows a piece, they should practice it slowly at first, making sure to get all the notes clear. Easier said than done!
This stronger type of playing will backslide between lessons, but if you regularly spend some time refocusing and reenergizing his roll to be louder and clearer, the good sound will develop.
Best of luck!