Microsoft, during the introduction of the Surface tablet

Microsoft’s Surface tablet has been on the market for nearly three months. So far, not so good.

Brent Thill, an analyst at UBS Investment Research, estimates that Microsoft sold just one million Surface tablets in the fourth quarter — half his original estimate of two million. Some other analysts estimate that sales were even lower.

By comparison, Apple sold three million of its newest iPads in the first three days they were on sale in November. It has sold more than 100 million iPads over all.

In a research note issued Monday, Mr. Thill said he believed sales of the initial version of the tablet, the Surface RT, suffered because it wasn’t sold in many retail stores. He added that the Surface RT, which costs the same as an iPad, had a tough time competing with Apple’s tablet. He was more optimistic about the Surface Pro, a high-end version for professionals that will be released this month, because it is likely to be a more compelling alternative to the iPad.

Rhoda Alexander, a display analyst at IHS iSuppli, said her firm estimated that Microsoft sold about 1.3 million devices running Windows RT, the mobile version of the Windows 8 operating system, in the fourth quarter. Windows RT is shipping on multiple devices, including the Surface RT, so sales of the Surface RT alone might be lower than one million, Ms. Alexander said.

“It’s certainly not a gangbusters start,” she said.

Despite the skepticism, Microsoft said it was increasing production of the Surface and expanding distribution to more stores.

“We’re pleased with the strong customer reception of Surface and are excited to continue to grow the Surface family,” a Microsoft spokeswoman said in a statement. “We’ve recently increased production and expanded distribution to additional retailer partners given the growing demand. We look forward to more customers getting hands-on time with the product as we continue to expand its availability in the coming months.”