Each flown parabola is different due to turbulent air conditions. In the phase between the 1.8 G force and the microgravity a very short phase with a negative G force appears. That negativ G force lifts-off the marbles.
After the marbles have lift-off the airplane flys further on its parabola. That causes the marbles to touch the rear wall inside the experimental area. So it is not possible to make the sculpture made out of marbles floating for longer.
more info on parabolic flight (german) on wikipedia