While low inflation might sound great, a never-ending shortfall might hurt the economy. Modest price increases can brighten the economic picture by allowing wages to rise without crushing profits. Janet L. Yellen, the Fed’s former chairwoman, often describes inflation as a lubricant on the wheels of the labor market: It keeps wages chugging along.

“I am concerned that inflation is running lower than I would expect, especially considering that now we’ve had sub-4 percent unemployment for a long time, we’ve had growth that’s surprised to the upside,” James Bullard, the president of the Federal Reserve Bank of St. Louis, said in an interview. “We’re on the wrong side, and it’s kind of going in the wrong direction.”

The breakdown leaves the Fed staring down an uncomfortable question. If officials can’t get that old chain reaction to work 10 years into an economic expansion, against a backdrop of tax cuts and high government spending, and with exceptionally low joblessness, will they ever?

The Fed’s chairman, Jerome H. Powell, has called weak inflation “one of the major challenges of our time.” In part to address it, he has led the Fed to embark on a yearlong review of its communications, tools and strategy. A major goal is determining what is reining in price gains and what can drive inflation back to the Fed’s target in a sustainable way.

Extra labor supply is one obvious culprit. Since 2016 at least some Fed officials have declared the labor market “at or near full employment.” But the job market keeps surprising them. Prime-age workers are hanging on to their positions for longer.

That has provided an unexpected source of new employees, enabling brisk hiring to persist without a run-up in wages and prices. Average hourly earnings have shown progress without rocketing up.