Which of the following is a common reason why balancing may fail?

Balancing may fail when the machine was not actually unbalanced because an adequate diagnosis is essential before taking corrective action. When an imbalance is presumed but does not exist, the efforts to balance the system become futile; adjustments made can inadvertently introduce new issues or exacerbate existing problems. This misconception can lead to wasted resources and time, as the root cause of vibration or performance issues might be misidentified.

In the context of the other choices, while they represent possible mechanical issues that could affect machine performance, they do not directly relate to the fundamental purpose of balancing. If the rotor is too heavy, it can indeed complicate balancing, but the failure would not stem from the act of balancing itself. Similarly, high machine speed can affect the dynamic behavior of a rotor, but it does not cause the balancing process to fail if an imbalance is present. Worn-out bearings may contribute to vibration but are not a direct cause of balancing failure if the machine was indeed unbalanced from the start. Thus, understanding the state of balance is crucial, which emphasizes why assuming an imbalance can lead to balancing failure.