The business of bras is broken.

The leading lingerie brand, Victoria's Secret, hasn't kept up with customer preferences. Shares of L Brands (LB), the parent company, are down 46% this year.

The brand still leads the women's undergarments sector, which includes bras, lingerie and underwear, according to the research group Euromonitor International.

But flashy fashion shows, push-up bras and celebrity models aren't drawing people in like they used to. Instead, women are clamoring for more products with a better fit.

One young online competitor, ThirdLove, announced last month that it had 1.3 million women on the wait list for its 24 new sizes, which range from a 28 to 48 band and AA through H cup, including half sizes.

In the first five days that the new sizes were available, ThirdLove said it sold $1 million worth.

Online stores such as ThirdLove, Lively and True&Co. are using tech and harnessing data to offer custom sizes and new products.

Simeon Siegel, a retail analyst with Instinet, said the recent trend of bralettes — bras with no padding or underwire that come in traditional sizes such as small, medium and large — helped crack the door open for online retailers.

"With the emergence of the bralette ... sizing changed dramatically," he said.

Bras come in a large range of sizes, and it can be tricky for women to figure out the right fit without trying one on. Because bralettes don't use specific band measurements or cup sizes, customers felt comfortable buying them online, Siegel explained.

Once women got used to shopping for bras on the internet, they began experimenting with more complicated fit, he explained.

"All of the sudden, the moat that had been dug very deep around Victoria's Secret started to deplete," he said.

Related: Victoria's Secret customers are buying bras online ... and that's a problem

Michelle Cordeiro Grant, founder and CEO of Lively, said her company focuses on how "women feel in products when they look at themselves," rather than how they want to be seen by significant others.

After customers complained that bralettes don't offer enough support, Lively unveiled the "busty bralette" in larger sizes. The new item sold out in a day, said Grant, and thousands of women signed up on the waitlist.

Heidi Zak, the co-founder of ThirdLove, said she began thinking seriously about lingerie after an unsatisfying experience at Victoria's Secret.

Related: Victoria's Secret owner is the S&P 500's worst stock

"I was walking around in the store thinking to myself, 'Why am I in here?'" she recalled. When she got home, she started researching the market. "I thought to myself, 'If there isn't something, I'll create it.'"

Zak said that for many women, bra shopping is an unpleasant experience. It's hard to find your size, and stripping down to get measured by a salesperson can be embarrassing.

"It's not a social shopping experience," she added.

She explained that last year, ThirdLove ran a beta test with 15 new sizes and sold out within weeks. "Between last year and this year there's been a lot of pent up demand for these sizes," she said, which helped build up the wait list to 1.3 million.

Both ThirdLove and True&Co. use an online fit quiz that asks about band, cup and breast shape to help women figure out the right bra size for them. Victoria's Secret's online fit quiz does not include questions about breast shape.

As more women use the quizzes, the companies can collect more data about their customers' needs.

Michelle Lam, the CEO and co-founder of True&Co., said her company has more than 130 million data points because of the quiz.

Victoria's Secret could benefit from a better loyalty program, said Oliver Chen, an analyst at Cowen. They "have to really rethink customer data," he said. "If you're running a big brand, it becomes harder for you to change."

L Brands did not return a request for comment. But the company is already making changes.

During a call with investors in May, Stuart B. Burgdoerfer, the chief financial officer of L Brands, said he was "not satisfied" with the recent decline in Victoria's Secret sales, and promised to improve.

Victoria's Secret has launched new products, like a line of bras with seams that don't show through clothes and better-fitting T-shirt bras.

"These bras are bringing in new and younger customers," said Jan Singer, the CEO of Victoria's Secret. Singer added, "we'll continue to evolve."