The number of women on corporate boards is barely budging.

Women hold less than one-fifth of board seats at S&P 500 companies, according to Catalyst, a research group that tracks executive women. Catalyst’s annual board census found that 19.9% of board seats were held by women last year, compared with 19.2% the year prior.

Progress toward boardroom parity is “painstakingly slow,” said Trina Gordon, the chief executive of executive search firm Boyden World Corp.

And the pace doesn’t appear to be picking up much. Just more than a quarter of open director positions last year went to women, with the remaining 73.1% going to men.

Some companies have increased female representation on their boards. Microsoft Corp. added Sandra Peterson, group world-wide chairman at Johnson & Johnson, to its directors in December, and Twitter Inc. last month announced that BET Networks CEO Debra Lee was joining its board. Fitbit Inc. earlier this month named women executives from Williams-Sonoma Inc. and Whole Foods Market Inc. to its previously all-male board.

Other firms, however, remain resistant. Shareholders of Restaurant Brands International Inc., which owns Burger King and Tim Hortons, recently voted down an investor proposal meant to boost diversity on its all-male board.

“RBI values its diverse and dynamic workforce and promotes a culture of equality and inclusion,” the company said in a statement, adding that the board believes its practices “support board diversity.”

Catalyst cautions that it can be hard to draw trends across years. For this year’s report, it analyzed the makeup of boards at the time of a company’s 2015 annual meeting. For the 2014 data, it looked at where companies stood that October.

According to Catalyst’s most recent analysis, fewer than one in six companies have boards with 30% women or more; about one in four boards have a lone female director. Boards without any women directors are becoming slightly less common. In 2015, 2.8% of firms’ boards were all male, as compared with 3.6% in 2014.

Ms. Gordon said that virtually every list of candidates her firm compiles for companies looking to fill board seats includes women.

“There are absolutely more women on the slate. Are they always getting selected? Clearly not,” she said.

Some companies truly are committed to diversifying their directors, but others just “give it a good lip service,” she added.

The state of women in corporate management isn’t much better than in the boardroom. Catalyst found that women held 4.2% of CEO jobs last year and represented just more than a quarter of executives and senior managers, barely up from a year earlier.

Having a female CEO usually means more women directors, too. A recent report from advocacy group 2020 Women on Boards analyzing 2015 Fortune 1000 companies found that 88% of companies with a female CEO have boards with at least 20% women directors. Some 42% of the total pool of firms in that analysis have reached that level of representation.