Iran’s revival as a crude-oil exporter appears to have stalled, seven months after Western sanctions over its nuclear program were lifted, casting uncertainty over the country’s willingness to cooperate with other producers on limiting output.

Iran’s production has taken on heightened significance in recent weeks as the Organization of the Petroleum Exporting Countries gets ready for talks next month on oil output. Iran has previously refused to consider joining fellow OPEC members in action to lift crude prices by curbing output until its exports and production reach presanctions levels.

But the country’s ability to reach presanctions levels above four million barrels a day are now in question. Iran has been pumping 3.85 million barrels a day this month, the country’s oil minister Bijan Zanganeh said Saturday, little changed from above 3.8 million barrels a day he cited in June. That is up from less than three million barrels a day from before sanctions were lifted in January but short of the country’s stated goals.

The International Energy Agency says Iran’s crude production actually fell in July, to 3.6 million barrels a day.

Iran’s crude-oil exports have also plateaued after nearly doubling from January to May. Mohsen Ghamsari, the head of oil marketing at the National Iranian Oil Co, told state media last week that the country was still short of pre-sanctions exports by 200,000 barrels a day.

Iran has told OPEC that it plans to participate in next month’s talks, national delegates to the cartel said, but the country hasn’t said it would soften its conditions for actually joining in production limits. Mr. Zanganeh confirmed Thursday he would attend the informal gathering, according to Iran’s oil-ministry news agency Shana. Iran and fellow OPEC member Ecuador talked Wednesday about stabilizing oil prices, Ecuadorean official said.

Iran’s stalled production “will complicate the negotiations,” said John Hall, chairman at U.K. consultancy Alfa Energy and longtime observer of OPEC. “It’s going to be difficult. The Iranians are not going back down.”

On the other hand, some within OPEC said, Iran hitting a natural ceiling on its output might be enough to persuade rival Saudi Arabia to join an output agreement next month. Saudi Arabia has pumped its own production up to record levels this summer and may be more inclined to make a deal next month if Iran seems weak.

A government oil official at a Persian Gulf nation said the situation could make it easier for OPEC members to mend fences. “Iran’s production seems to be stuck at a certain level and not rising like before,” the official said.

Iran has faced two significant obstacles in its quest to return to pre-sanctions production levels: a lack of foreign investment and its own unwillingness to undercut rivals on pricing.

Lingering American sanctions on Iran over terrorism, human rights and weapons continue to make it difficult for Iran to do business with European crude-oil buyers, whom the Islamic Republic had been counting on to lift its exports. The American sanctions ban dollar transactions with Iran, requiring European companies with U.S. business to go to great legal lengths to do deals there.

“Other exporters took Iran’s place during the sanctions,” said Erfan Ghassempour, an adviser to foreign oil companies in Iran. “Now it will take time for Iran to take back its old friends.”

A case in point is Italy, which was Iran’s largest oil importer in the European Union before the bloc embargoed its crude in 2012. Italy didn’t buy any Iranian crude in the first five months of the year, according to statistics released by industry body Unione Petrolifera last month.

Sardinia-based refiner Saras SpA was forced to delay its first Iranian cargo to June “because of the difficulties of establishing financial and banking lines to operate effectively with Iran,” its general manager, Dario Scaffardi, told analysts on Aug. 1. An official at the company said most banks had initially refused to handle the transactions because they feared they could violate remaining Washington’s sanctions.

Obama administration officials have repeatedly told European banks they can deal with Tehran.

Despite the banking issues, Iran has refused to discount its oil against other producers, according to European refiners and Iranian officials. Saudi Arabia has aggressively cut the price of its oil, and Iran has matched those cuts—though they have never gone deeper to make up for the hassle of obtaining financing to buy its crude.

“In this context, Iran is just too expensive,” an Italian refinery official said.

An Iranian oil official said Tehran doesn’t want to sell its oil off on the cheap, seeing it as a precious and finite national resource. Iran also has a more diverse economy than many of its rivals, with petroleum revenue accounting for about 29% of its national budget.

Reaching presanctions levels is a key goal of moderate Iranian President Hassan Rouhani as he tries to woo the opinion over the merits of the international deal that lifted restrictions on Tehran’s exports in January but also placed curbs on the country’s nuclear program.

That could prove challenging. Apart from competition from rivals, underinvestment in Iranian oil fields is also capping exports, said Hamid Hosseini, a board member of Iran’s oil exporters’ association.

Homayoun Falakshahi, an oil-industry analyst at consultancy Wood Mackenzie, said that oil production initially ramped up faster than expected because pressure was high in wells that had been shut for years. But Iran will struggle to pump more because the fields have high depletion rates and now need significant investment at a time when Iran remains cash-strapped, said Mr. Falakshahi, who visited key oil-producing facilities in Iran this spring.