Can you tell me more? The scale only registers the target weight after the player drops in - ordinarily this is enough time for everything to settle, and it should be solvable even with one (or both) of the starting rocks being kicked off or one of the junk rocks rolling on. It's intended that two rocks usually land on the scale with the snacks, though.
And thank you! Aimed it to be the equivalent of a very short panic-driven comedy.