Problems with the enrichment technology could reduce the international pressure over Iran's civilian nuclear programme, which the United States claims could be cover for a weapons project
On Tuesday, Iran rejected calls for an immediate response to an offer of incentives to suspend uranium enrichment.
Washington responded by reminding major powers that they had resolved to take Tehran to the UN Security Council if it did not give a timely response to the offer.
In April, Iran enriched raw uranium to the level needed to fuel nuclear power plants for the first time, but far short of that suitable for a warhead. It began a second round of feeding uranium into centrifuge enrichment machines on June 6.
Some Western diplomats in Vienna accredited to the UN nuclear watchdog agency said Iran's cascade of 164 interconnected centrifuges at its Natanz pilot plant had been plagued by technical problems which have affected production.
"We have been told of problems from people in a position to know. It's a slowdown in the process although we haven't been able to quantify it yet," one diplomat said.
A second diplomat said plans to expand the enrichment programme had been suspended. "We have heard ... that plans for a second and third cascade of 164 are on hold and that the attrition rate in the first cascade is relatively high," he said.
An EU diplomat said the problems arose from poor quality control and impurities in Iranian-made "UF6" uranium gas poured into centrifuges. "Their success in April was apparently done with higher-grade, imported Chinese UF6," he said.
The first diplomat said there were unconfirmed reports that the first cascade, the basis for Iranian plans to install 3,000 centrifuges by next year, had a "failure rate of up to 50%".
"The reasons for the delays are definitely not political, that is, it's not like it's an Iranian goodwill signal as they go into negotiations with the EU," he said.
Centrifuges are prone to breakdown due to excessive vibration or pressure and temperature changes.
Iran would have to run thousands of centrifuges continuously for months or years to enrich uranium in sufficient quantities for a nuclear warhead.