When changes in laws result in reduced sentences for crimes, people currently in prison for those offenses don't necessarily see mercy. So when the Supreme Court in 2015 declared a federal mandatory minimum sentencing trigger unconstitutional, it didn't automatically mean prisoners already sentenced would see their time in custody reduced.
In that case, Johnson v. United States, the justices ruled 8–1 that a provision of the Armed Criminal Career Act of 1984 was unconstitutionally vague. That guideline imposed a 15-year mandatory minimum under certain circumstances, one of which was a third "violent felony." But the law's definition of a violent felony included "conduct that presents a serious potential risk of physical injury to another." This overly broad description resulted in the government declaring that simply being a felon in possession of a gun (without using it to commit a crime) triggered this enhanced sentence.
The Supreme Court ruled that this clause "fails to give ordinary people fair notice of the conduct it punishes" and struck it down. But a second case had to be filed with the Supreme Court to determine whether the change should apply to people currently serving time. In April, the Court ruled 7–1 in Welch v. United States that the elimination of the mandatory minimum sentence should be retroactive, giving federal prisoners the opportunity to petition for relief.