 This could either be due to using the wrong kind of rice (long grain rice, jasmine rice, basmati rice, etc. are not sticky enough to make a rice ball with), improper cooking of rice (it should not be too dry/hard OR too sticky/gluey) or using a filling that's so oily or watery that the oil/water seeps out into the rice, making it fall apart.  This could either be due to using the wrong kind of rice (long grain rice, jasmine rice, basmati rice, etc. are not sticky enough to make a rice ball with), improper cooking of rice (it should not be too dry/hard OR too sticky/gluey) or using a filling that's so oily or watery that the oil/water seeps out into the rice, making it fall apart. 