LIMA, Peru—The world’s policy makers are searching for new engines of economic growth. They are finding few that aren’t sputtering.

At the International Monetary Fund’s annual meeting here, top finance officials in recent days confronted an increasingly troubling reality as they searched for ways to pull the global economy out of a rut.

“This is a pretty unforgiving environment,“ said Bank of England Governor Mark Carney. ”It’s not a strong global economy.”

The most powerful engines since the global financial crisis—China, Brazil and other emerging markets—are slowing or contracting as they strain their capacity to expand without major overhauls to their economies. Big advanced economies are stuck with weak growth and little prospect of accelerating sharply.

International officials are bracing for the implications of years of sluggish growth. Emerging economies that managed to dramatically reduce poverty in the past decade now are struggling to maintain social gains, while policy makers in developed countries struggle to enact policy overhauls to breathe new life into their economies.

“There’s a great sense of unease,” said Tharman Shanmugaratnam, Singapore’s deputy prime minister and the former chairman of the IMF’s policy committee. “That sense of unease explains why globally, almost everywhere, investment is much weaker than we’d expect.”

Wide-ranging uncertainty is fueling volatility in bond, equity and currency markets around the world.

It is unclear whether China’s opaque economy will transition smoothly to a new growth model or face a sharper deceleration.

Meanwhile, officials are divided about whether the U.S. Federal Reserve’s plans to raise rates for the first time in nearly a decade will be premature, smothering a modest recovery, or too late to stem a dangerous buildup of risk across the financial system.

The impact of plunging commodity prices on the global economy is blurry: Promised gains for importers have failed to materialize, and exporters’ growth rates are tumbling with shrinking revenues.

The IMF last week again downgraded its outlook for the global economy, putting growth at the slowest rate since the 2008-2009 financial crisis. Officials are still counting on emerging economies to step up their game.

“If there’s going to be strong growth in the global economy, the engine of growth will be emerging markets,” said David Lipton, the IMF’s No. 2 official.

China, which in the past decade expanded at breakneck speed, gobbling up iron ore, oil and other commodities from around the world, is now set to expand at a much slower pace as it moves away from reliance on exports and toward more dependence on domestic consumption. The lower speed means raw materials are now in large supply and countries that relied on exporting them to Chinese buyers are selling less and at much lower prices.

The commodity price drop and plummeting demand from the world’s No. 2 economy has exposed the underlying weaknesses in many of those countries. Governments have failed to restructure their economies to make them more competitive, productive and efficient. Such changes would have raised their potential to expand, attracting investors. Instead, the IMF has downgraded its estimate of those nations’ growth prospects.

“What’s exceptional about emerging markets’ policy-making at the moment is that very, very few countries have a strong commitment to that kind of structural reform,” said David Lubin, head of emerging-market economics at Citibank. In the absence of such restructuring, the only thing that is left is exchange-rate depreciations to levels that jump-start exports again, he said.

In many countries, policy makers are pinning hopes for economic growth on infrastructure investment, but cash-strapped governments need private-sector investors to finance projects. With the Fed close to raising rates, it is unclear whether global investors will be willing to jump into higher-risk countries.

With the tumble in commodity prices, many emerging-market economies that relied on exports will first have to clean up their government finances before being able to attract private capital needed to spur growth.

“We have to fix the fiscal situation to put the country on a growth path,” said Ariosto de Carvalho, who manages foreign reserves at Brazil’s central bank.

Brazil’s situation shows the difficulties of countries struggling to overhaul their balance sheets and restructure their economies.

President Dilma Rousseff, whose party has lost support amid a major corruption scandal, has struggled to raise taxes and cut spending in the face of strong resistance in the country’s congress.

Brasília is far from alone. Nearly two years ago, the Group of 20 largest economies vowed to roll out scores of new policies that would boost global output by two percentage points of GDP. Those promises have been hobbled by politics at home, and many official acknowledge the group is well behind schedule.

Even if governments successfully revamp their economies, those efforts will take years to bear fruit.

Colombian Finance Minister Mauricio Cardenas is pushing for a more balanced budget as a way to bolster investor confidence. He wants to diversify the country’s exports out of commodities and into higher-value goods.

“We have about three years to do the transition,” he said.

Ilan Goldfajn, chief economist at Itau Unibanco, Brazil’s largest private-sector bank, said it could take up to five years for the U.S. and other developed countries show steady, strong growth, with emerging markets following suit.

The golden years of strong growth are a relic of the past, Mr. Goldfajn said. “You will tell your grandchildren about them.”