Tesla is getting overwhelmed by demand and isn’t responding to customers quickly, Chief Executive Officer Elon Musk said, acknowledging the pitfalls of progress his company has been making in producing more electric cars.

“Due to a large increase in vehicle delivery volume in North America, Tesla customers may experience longer response times,” Musk wrote in a tweet Wednesday. “Resolving this is our top priority.”

Musk had responded minutes earlier to a customer who said the process of taking delivery of a Model 3 sedan had been a “nightmare” and that his delivery adviser hadn’t been responding to emails. The CEO apologized and said he was “working on this exact issue right now.”

Tesla started taking orders for the Model 3 in March 2016 and quickly accumulated hundreds of thousands of reservations. But the company kept the vast majority of those prospective customers waiting until late last year as it struggled to ramp up production. While the carmaker touted the vehicle’s $35,000 starting price, it still hasn’t started delivering the cheapest version of the sedan.

Tesla charged $1,000 for a Model 3 reservation and then $2,500 to configure their vehicle online and confirm their order. All reservation holders in the U.S. and Canada were invited to design their car and select options in late June.

The company announced days after opening the floodgates for orders that it achieved a target to produce 5,000 Model 3s in the last week of June. That late-month surge contributed to production exceeding deliveries for the quarter by more than 10,000 units.

Last month, Tesla forecast that it would produce as many as 55,000 Model 3s in the third quarter, from 28,578 in the prior three months, and said deliveries should outpace output.

Musk suggested there was plenty of momentum toward that goal in an email to employees on Friday, which was republished on Tesla’s blog. He wrote that the company was “about to have the most amazing quarter in our history, building and delivering more than twice as many cars as we did last quarter.”

On Tuesday, Musk tweeted that the company would limit the number of paint colors available so that production would be simpler for the company.