Once a defendant has exhausted his or her right to a direct appeal, one of the few remaining ways to challenge a sentence is to move the court to modify the sentence based on "new factors." A new factor is a fact or circumstance highly relevant to the issue of sentencing which was not known to the Court at the time of the original sentencing, either because it was not then in existence or because it was inadvertently overlooked by all of the parties. Whether something is a "new factor" thus depends on what the sentencing court actually considered to be important when imposing sentence.
In addition to showing a new factor, the defendant must convince the court that the new factor should make a difference in the sentence imposed under all the circumstances. This would include taking into account the defendant's conduct since the time of the original sentencing as well as any other bad (or good) conduct by the defendant that comes to light after the original sentencing.