The Tesla Model 3 remained the best-selling electric car in the United States last month by far, according to estimates from website Inside EVs.

Tesla sold 10,050 Model 3s in April, the website said; the runner-up, the Toyota Prius Prime, notched only 1,399 sales.

But Model 3 sales were flat compared with the previous month. That presents another challenge for Chief Executive Elon Musk, who pledged that Tesla sales would get a significant boost this quarter.

Tesla reports quarterly figures, not monthly ones, but Inside EVs’ estimates have proved fairly accurate.

The 10,050 Model 3s it said Tesla delivered last month was a big jump from the same month last year, when the car was hitting the market amid production problems and 3,750 were delivered. But April deliveries were down from March 2019’s 10,175.

Sales for the older Model S sedan were down significantly in April, to 825 from 2,225 in March. In April 2018 Tesla sold 1,250, according to the estimates.

The Model X SUV had 1,050 sales in April, Inside EVs said, down from 2,175 in March and a slight uptick from April 2018’s 1,025.

Tesla tends to pack deliveries into the last month of each quarter, so April-versus-March comparisons don’t necessarily indicate a fall in demand. However, Tesla had planned to be selling half a million Model 3s a year by now, so the current sales figures are cause for concern.

Tesla outsells other electric-car makers, but it currently can’t sell its cars at a profit.

Other automakers are selling many of their electric cars at a loss as they conform with government mandates that aim to get more zero-emission vehicles on the road.

Like all electric-vehicle makers, Tesla benefits from government subsidies, in the United States and internationally. But unlike many other automakers, Tesla has no profits from internal-combustion vehicles to make up for losses it racks up from selling electric vehicles.

Batteries remain the biggest cost in electric vehicles. Musk had vowed to bring the cost down low enough to make profits, but has yet to prove he can do so.

Building up scale is one way to do that. But Tesla delivered only 63,000 vehicles worldwide last quarter, a drop of 31 percent from the previous quarter.

Musk told analysts recently that he expects to deliver at least 90,000 in the current quarter. Reaching that goal will depend on how well Tesla cars sell in Europe and China.