ATHENS—Greece’s economic recovery is proving elusive, challenging the forecasts of the country’s government and foreign creditors still counting on growth reviving this year.

The International Monetary Fund said last week  that the economy is stagnating, in the first admission from creditors that Greece’s recovery is off track again. Growth will only restart next year, the head of the IMF’s team in Greece said on a conference call with reporters, without offering details.

Of particular concern is that exports, which are supposed to lead Greece out of trouble, are on a slow downward trajectory, hampered by capital controls, taxes and a lack of credit.

“There is no chance we will see a rebound unless we see some bold political decisions that would introduce a more stable business environment,” said Dimitris Tsakonitis, general manager at mining company Grecian Magnesite.

The bailout agreement between Greece and its German-led creditors assumes rapid growth from late 2016 onward, including an official forecast of 2.7% growth in 2017. Private-sector economists believe next year’s growth could be closer to 0.6%.

Weaker growth would undermine the budget, likely leading to fresh arguments with lenders about extra austerity measures.

Greece is still grappling with the measures it has already agreed to. Late on Tuesday the country’s parliament approved pension overhauls and other policy changes that have been delayed for months, holding up bailout funding.

Greek government officials are sticking to their view that the economy is on the cusp of growth. “We are at the turning point at which we can we say with certainty that we are leaving the recession behind us,” Economy Minister George Stathakis told supporters of the ruling left-wing Syriza party Sunday.

The economy will get a push from investors as of the end of the year, when lenders are expected to provide some debt relief and the country qualifies for a European Central Bank bond buyback program, Prime Minister Alexis Tsipras said in an interview with The Wall Street Journal last week.

Data so far show at best minimal improvement. Unemployment is creeping down, but remains at around 23%. Retail sales and tourism earnings have been disappointing. The number of business insolvencies in the first half of the year reached the highest level in three years. Industrial production data have looked brighter, but economists say sentiment surveys don’t bode well for coming months.

Stagnant exports are the biggest worry. Greece’s bailout plan relies on shifting the economy away from government spending, toward exports and related private-sector investment.

Data from the Bank of Greece, released last week, showed exports in the year to July were 7.1% down from the same period last year. Even without the volatile shipping and oil sectors, exports were down 2.2%.

Starting in 2009, exports enjoyed several years of growth, providing a rare bright spot. But export data have been persistently weak since July 2015, when Greece had to impose capital controls to stop bank runs while it held a referendum on creditors’ bailout terms.

The government has loosened controls but given no indication of when it might lift them completely. To pay foreign suppliers, Greek companies have to undergo a bureaucratic process involving the finance ministry and banks.

The process has been gradually streamlined, but the impact lingers. Nearly all foreign suppliers to Greek companies demand full payment in advance, in case controls get worse and cause long delays in getting paid.

At Grecian Magnesite, the combination of capital controls and lack of credit has led to declining sales, Mr. Tsakonitis said.

The company has to pay up-front for raw materials and spare parts from abroad, Mr. Tsakonitis said. Because it can’t meet all the payment terms some foreign suppliers demand, it has to turn away some business opportunities.

The company offers its own foreign customers up to 120 days to pay for its product. But because it has to pay earlier for its own supplies, it needs credit—which Greece’s battered banks aren’t offering. Grecian Magnesite can only get limited overdrafts from banks, and has to pay high interest of 6.5% on them.

“It is like having a noose around your neck,” Mr. Tsakonitis said. Lack of liquidity means the company also has to pay Greek suppliers late, he said, causing the company to miss out on price discounts it could otherwise get.

Greece’s latest increases in taxes and social-security contributions this summer are weighing down even strong businesses. The government says it is aware of the pain taxes cause but has limited ability to cut more spending to create the savings lenders seek.

Security-systems provider Olympia Electronics says it has to pay an extra €40,000 ($45,000) a year to pension funds for its 160 workers under changes passed in June. That increase came after Greece’s corporate income-tax rate rose to 29% from 26% in January. Also, Greek companies now have to pay 100% of their estimated annual taxes up front, versus 80% previously. Next in line is a higher dividend tax that comes into effect in 2017.

Olympia’s chief executive, Dimitrios Lakasas, said the company would have hired some new staff this year, because revenues have been growing. But the latest hike in pension contributions prompted it to put off expansion plans.

“If we operated in a friendlier environment, we would have been a lot more aggressive in growing the business,” he said.