When Mordechai told Esther that two of the King's guards were plotting to kill him, she went right away to tell the king directly.
Yet when Mordechai later asked her to approach the king to save all the Jews, she resisted and said she couldn't approach him without having been summoned.
I can think of a couple plausible explanations, but I don't know if any of them is correct:
Esther just happened to be summoned before the king in the first instance. (But if so, why didn't the Megillah tell us this, since in the later instance it tells us just how dangerous it is to go without being summoned - wouldn't this be yet another instance of hidden Divine assistance?)
Esther was admonished for approaching the king without having been summoned in the first instance. (This could explain her rather lengthy explanation later, not to mention her emphasis that, "everyone in the kingdom knows that you can't do this.")
A twist to the second scenario above: the first instance resulted in a change in the law (and/or a widely publicized announcement of the law).
It was simply due to the nature of the assignment: the first being to save the king, the second being to save herself and her people.