When it comes to studies conducted on whether or not a supplement can provide a benefit to sports and exercise, the first batch of studies is almost always done on untrained or lightly trained people. There is a reason for this, and that is because this group of people are going to get benefits anyways when they start to exercise and any interaction of the supplement is going to be more prominent to the researchers. It is only later that researchers look into elite athletes since, not only are they more uncommon, but tend to be more resistant to dietary supplements benefitting performance.
It is not uncommon for a supplement to benefit the novice but fail in the elite; does creatine, the king of ergogenics, fall into this category?
Creatine supplementation has indeed shown benefit in athletes considered to be on an elite level and has been shown to do so in; soccer, volleyball, wrestling, swimming, and more. There are a few times where creatine supplementation does not benefit elites however, usually on measurements that aren't related to bursts of power.
Finally, it does seem that the magnitude of benefit is lessened a bit with studies in elite athletes noting minor anti-fatigue effects and increases in strength which is in contrast to supplementation promoting large strength gains in the novice athlete.