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The amount of 99Mo on the column does not directly affect generator yields in the same way that the other options do. Generator yield is primarily influenced by the conditions surrounding the processing and elution of the radioisotope, such as the timing and integrity of the generator system.

Time since last elution plays a critical role; as time passes, the amount of 99mTc available for elution decreases due to decay and possible adsorption losses. Therefore, shorter intervals between elutions generally result in higher yields.

Column cracks and air leaks introduce significant risks of contamination and loss of radioactivity, impacting the efficiency of the generator. Cracks can prevent the proper flow of solutions, and air leaks can lead to the introduction of oxygen, which may oxidize the pertechnetate and decrease yields.

In contrast, while the amount of 99Mo on the column is important to ensure sufficient starting material is available, it does not influence the immediate yield dynamics during elution unless it falls below a critical level. Therefore, it is the least direct factor affecting generator yields compared to the others mentioned.