Like any 1-year-old, the First Step Act’s first steps to reform the criminal justice system have been significant, if not always sure and steady.

The law’s first anniversary fell just before Christmas. By limiting mandatory minimum sentences, retroactively reducing sentences and expanding rehabilitation programs for federal inmates, the bipartisan achievement marked a major diversion from the mass incarceration mania that dominated American law enforcement for decades.

But moving from the intent of the law — trumpeted by President Trump, Democrats and Republicans, liberals and conservatives — to the reality of implementation is akin to a baby learning to walk. Celebrated progress is mixed with inevitable stumbles.

At the center of the change is the U.S. Bureau of Prisons, which currently incarcerates about 175,250 inmates. Although the law applies only to the federal government, Uncle Sam’s policies often influence what happens in the states, where the bulk of the nation’s 2 million prisoners are held.

“While sentence reductions have been approved by judges, the Department of Justice (DOJ) has attempted to block hundreds of eligible beneficiaries,” according to a report by the Sentencing Project, an interest group that has long pushed criminal justice reform. “There has also been a problematic rollout of the risk and needs assessment tool to determine earned-time credit eligibility and limited programming for rehabilitation.”

That appraisal contrasts with self-praise from bureau Director Kathleen Hawk Sawyer. She didn’t agree to an interview, but in a statement, she said her agency “has made great progress in implementing” the law.

It led to sentence reductions for more than 1,500 inmates as of October, she told the House Judiciary Committee in the fall. Nearly 100 prisoners, generally old or ill, received compassionate release, with 328 approved for an elderly offender pilot program that placed 242 offenders in home confinement. Within seven months of the law’s implementation, she said, the agency released more than 3,000 inmates because of changes to good-conduct time calculations.

Although Hawk Sawyer said prisons officials have long believed that inmate “release preparation begins on the first day of incarceration,” prisoner advocates see a different side of the agency.

“The BOP has a long history of acting in ways that result in lengthier and less productive terms of incarceration despite the obvious will of Congress,” David E. Patton, executive director of the nonprofit Federal Defenders of New York, said in a statement.

“For decades the BOP took an unreasonably restrictive view of good time, resulting in thousands of years of additional overall prison time,” he said. “For decades it refused to exercise the authority given to it by Congress to release incarcerated people who were terminally ill, infirm, or otherwise suffered from extraordinary circumstances . . . and for decades it has not provided enough vocational, educational, mental health, and substance abuse programming despite abundant need and lengthy waitlists.”

Participation in those programs can help inmates secure early release from incarceration, but only if slots are available

Pointing to Justice Department data, Patton said waitlists include 25,000 prisoners for prison work assignments, 15,000 for vocational and educational training and 5,000 for drug treatment.

Almost half the bureau’s prisoners complete no programs, more than half don’t get needed drug treatment, over 80 percent haven’t taken technical or vocational courses, and over 90 percent have no prison industry employment, according to the data.

Program participation is an element in an assessment system that classifies inmates’ risk of recidivism. That assessment affects their chances of early release.

One controversial element of the Trump administration’s implementation of the law is the Justice Department’s selection of the conservative Hudson Institute to select members of an Independent Review Committee, which advises on the assessment system.

While the Hudson Institute took no institutional position on the First Step Act, the organization did publish a 2016 article on “Why Trump Should Oppose ‘Criminal-Justice Reform.’ ”

Andrea James, formerly a criminal defense attorney and federal inmate, called on the Justice Department to “create a community oversight board that truly represents the political spectrum and makes a place for formerly incarcerated people to join the conversation.” Prisoners and formerly incarcerated people, James — executive director of the National Council for Incarcerated and Formerly Incarcerated Women and Girls — told Congress, “must have significant input in designing the recidivism assessment instrument.”

Hawk Sawyer acknowledged her agency’s “less than stellar performance” in some high-profile instances and promised to address the problems. Among them is understaffing.

“The BOP is struggling to fulfill the requirements of the Act as the Bureau is still more than 4,000 positions short,” Shane Fausey, president of the American Federation of Government Employees Council of Prison Locals, said by email. He complained of “abusive overtime and mandatory double shifts,” adding that requirements of the First Step Act have worsened the crisis.

Hawk Sawyer put the understaffing at over 3,700 vacancies and said resolving that “is among my highest priorities . . . but doing so will take time.”

Correctional officers aren’t the only people in prison with overtime problems. Inmates doing less time is a key point of the First Step Act.