While EV company Tesla has begun to increase the number of Model 3 vehicles it can make, it’s still struggling to hit its own numbers.

In a release on Wednesday afternoon, the company run by billionaire entrepreneur Elon Musk said it plans to hit a milestone of making 5,000 Model 3 vehicles per week by the end of the second quarter of this year.

But late last year, Tesla said it planned to hit that 5,000-car-per-week production rate by "late Q1." Previously, Tesla was trying to manufacture Model 3s at that rate sometime in 2017.

The new forecast puts Tesla closer to six months behind its original estimate for ramping up Model 3 production. The car is the company’s first lower-cost, mass-market-oriented electric vehicle, and hundreds of thousands of customers have reserved one.

The recurring delays, while not unexpected, highlight how difficult it is for Tesla to predict its own manufacturing bottlenecks and issues with suppliers. Musk has long said that production of new Tesla cars occurs like an “S-curve,” where initial manufacturing is low and slow; when an inflection point is reached, the production speeds up dramatically.

But at what point Tesla hits that inflection mark has been notoriously difficult to predict.

Tesla said on Wednesday it shipped 1,550 Model 3 cars to customers in the fourth quarter of 2017, with another 860 Model 3 cars in transit to customers. In total, Tesla produced 2,425 Model 3 cars in the quarter.

The company said it’s just now starting to hit its stride making the Model 3. Tesla said in its release that in the last seven working days of the fourth quarter it made 793 Model 3 cars.

The company said it has made “major progress” toward eliminating Model 3 production bottlenecks. That end-of-the-quarter rate extrapolates to “over 1,000 Model 3s per week,” according to Tesla. The company expects to hit a rate of 2,500 Model 3s per week at the end of the first quarter.

It’ll take Tesla another three months to double that number.

The more gradual ramp is so the company can “focus on quality and efficiency rather than simply pushing for the highest possible volume in the shortest period of time," according to the release.

Tesla (and by extension, Musk) has been accused in the past of being so aggressive on shipping cars that the quality of some early cars suffered. Some of the initial Model S and Model X cars had software issues and hardware glitches.

Last November, Tesla revealed that one of the big problems with manufacturing the Model 3 was the battery module assembly line at the Gigafactory outside of Reno, Nevada. That’s where battery cells are packaged into modules and four modules are packaged into battery pack.

Musk and Tesla CTO JB Straubel have been spending much of their time working on getting the Gigafactory to produce battery packs more quickly in an automated fashion. Musk even said that he camped out on the roof of the Gigafactory while working on the production problems.

Tesla didn’t comment on what part of the Model 3 production process was causing problems for the company in the fourth quarter.

Beyond the Model 3, Tesla’s overall car deliveries were on target for the fourth quarter and the year as a whole. The company said it shipped 101,312 cars combined in 2017, which was up 33 percent over 2016, and in line with expectations.

In the fourth quarter, Tesla shipped a total of 29,870 cars and made 24,565 cars across all models. Tesla said it was its “all-time best quarter” for combined Model S and Model X deliveries.  

At the time of publication, Tesla shares had dropped 2.01 percent in after-hours trading to $310.10 per share.