Scarce and finicky public chargers are among the biggest reasons people hesitate to buy electric cars. So when Elon Musk, the chief executive of Tesla, agreed last year to open the company’s well-regarded Supercharger network to vehicles from other carmakers, many drivers and industry experts celebrated the decision.

But more than 12 months later, Tesla’s network, with nearly 30,000 fast-charging plugs in the United States and Canada, remains largely inaccessible to most people who don’t drive Teslas because of software delays and hardware shortages.

The delays have fueled speculation that Mr. Musk was having second thoughts about opening up Tesla’s network, possibly because he was worried that access would help other automakers sell battery-powered models and lure customers from Tesla, which has suffered from declining sales.

Tesla eased those fears a bit on Friday when the company’s charging unit posted on X that it had stepped up production of a crucial piece of hardware: adapters that drivers of Ford, Rivian and other car brands need to connect to Tesla chargers.

A Tesla factory in Buffalo is producing 8,000 of the adapters per week, the company said, noting that outside suppliers are also producing the part. Still, it is unclear how fast those adapters would reach electric vehicle owners.

Tesla did not respond to a request for comment, and the other automakers have been reluctant to speak in detail, apparently because they do not want to antagonize Tesla and Mr. Musk.

The slow rollout raises questions about the decision that almost all major carmakers operating in the United States made to abandon the Combined Charging System, the standard that most of them used previously, and adopt the North American Charging Standard developed by Tesla. The switch made them vulnerable to the whims of Mr. Musk, who frequently changes corporate strategy and tactics in ways that can surprise even his employees and supporters.

Tesla built the Supercharger network to encourage sales of its own vehicles. By opening up the network, Tesla can make money from drivers of other car brands, who pay per kilowatt-hour to charge. The electric car company also makes money from selling adapters to other automakers.

But Tesla risks alienating its own customers, who will lose exclusive access to the chargers.

Tesla’s opening up of its chargers to other automakers was meant to be a three-stage process.

First, Tesla and each automaker would update the software on Tesla’s chargers and the other company’s cars so they could work with each other. Second, Tesla would make and supply adapters to allow other cars to connect to its chargers, which use different plugs. The final step is supposed to happen next year, when most automakers plan to start installing Tesla plugs on the new cars they assemble, eliminating the need for an adapter.

So far only two car companies have advanced past the first stage with Tesla — Ford Motor and Rivian. General Motors had said it expected to complete the software coordination with Tesla this spring but now says it will happen later this year. Other automakers are expected to follow G.M.

But even most drivers of electric Ford and Rivian models do not yet have access to Tesla chargers because the companies have not received enough adapters from Tesla.

It probably hasn’t helped that, in April, Mr. Musk abruptly dismissed nearly all of the 500 people responsible for the Supercharger network, including Rebecca Tinucci, who led the charging division. (Tesla has since rehired some of the employees. This month, Ms. Tinucci became the head of sustainability at Uber.)

“When Elon fired his whole Supercharger team I said, ‘This doesn’t look good,’” said Mike McMahon, a retired airline pilot who lives in Florida and is waiting for a free adapter from Ford that he ordered in February.

There is nothing to prevent Ford and Rivian from buying adapters from other suppliers, but it may not be easy to find firms with the manufacturing capacity and expertise. Adapters from other firms would have to go through an extensive testing and approval process.

Ford told owners of its Mustang Mach-E sport utility vehicle and F-150 Lightning pickup last month that the adapters were delayed by several months because of “ongoing supply constraints.” Ford, which had promised free adapters to owners of its electric vehicles, declined to comment further.

Rivian, which makes electric pickups and S.U.V.s, said “a challenging supply chain position” had led to adapter shortages. In a statement, the company added that it was “ramping up adapter supply as quickly as possible.”

“We are working closely with Tesla to support the industry’s overall transition” to the plugs used by Superchargers, Rivian said.

But having adapters will not be enough for drivers of electric cars not made by Tesla, Ford or Rivian. Every other automaker still needs to work with Tesla to make software updates for the cars to use the company’s chargers. That first step could drag on for months at the current pace.

G.M. has not said when its customers will be able to use Tesla chargers. “G.M. continues to work in good faith with Tesla to finalize an agreement that offers a seamless Supercharger network experience for our customers,” the company said in a statement for this article.

Besides frustrating owners, the Tesla charging bottleneck may discourage electric vehicle sales, which have been growing more slowly than a year ago. Tesla’s Superchargers are widely viewed as the most reliable, and they are often in places where other chargers are scarce, assuring drivers with access to them that they won’t be stranded with an empty battery.

About 100 Supercharger sites are equipped with adapters allowing them to work with non-Tesla vehicles. But that is a small fraction of Tesla’s network of more than 2,500 charging stations in the United States and Canada. Each station can have a dozen or more charging ports.

Behind the scenes, technical experts from automakers and other interested parties have been working to adapt Tesla’s proprietary technology so it works flawlessly with other electric vehicles. That process is often fraught, acknowledged Oleg Logvinov, chairman of the Charging Interface Initiative North America, an industry group whose members include Ford and Tesla.

“Transition is painful,” Mr. Logvinov said.

But once the problems are worked out, he added, “it’s hugely beneficial because it leads to a much higher adoption rate.” He noted that there had been competing technical standards in the early days of mobile phones, and that sales had taken off after the industry whittled them down.

Amid slumping car sales, Tesla has also slowed construction of new Supercharger stations, a blow to the Biden administration’s effort to encourage expansion of the charging network and promote electric vehicle use. The administration and many environmental groups view electric vehicles as crucial to reducing the greenhouse gas emissions that are warming the planet.

Last year, Tesla installed almost 6,000 fast-charging plugs in the United States, far more than companies like ChargePoint, EVgo and Electrify America, according to Rho Motion, a research firm. Tesla installed 2,750 during the first half of this year, but the pace has declined steeply since then, the firm said.

Some Ford and Rivian owners have gotten tired of waiting and bought adapters from other suppliers whose products are not endorsed by automakers. Their experience suggests that access to the Tesla network makes road trips much easier.

Reid Hester, a retired clinical psychologist who lives in California and owns a Mustang Mach-E, bought an adapter online for a little more than $200.

The Charging Interface Initiative, the group led by Mr. Logvinov, warns that adapters not endorsed by manufacturers may be unsafe. But Mr. Hester said his worked well. Having Tesla’s network as an option “just reduces your range anxiety,” he said.