If you can’t find your marble, it is most likely on the lowest level. To check it, shift all sliders up and hold the box vertically as shown below. Tilt it right and left – you will see the marble in the holes.
To release the marble, turn the box upside down and tilt it left and right slowly until it comes to the middle. Then turn over the box – the marble will roll out onto the plastic surface.
To continue the track correctly, bring the marble to the 5th cross.