By Sharon Terlep

A month after returning too-small yoga pants he had bought his girlfriend, Mark Quinn still hadn't gotten his money back from Lululemon.

Like other annoyed online shoppers, the 55-year-old executive from Boynton Beach, Fla., called and emailed the company to no avail. "You just couldn't get through to anybody, it was torture," Mr. Quinn said, until he finally got through and received his refund.

As the coronavirus pandemic accelerated the shift to online shopping, customer-service operations for retailers from Macy's Inc. to Wayfair Inc. are overwhelmed by shoppers frustrated by delayed shipments and sluggish refunds for returned goods.

Some shoppers are running up against hourslong wait times to be able to talk with someone, online chats where no one answers, and unreturned calls and emails. Even angry tweets, often a surefire way to get a company response, sometimes sit unaddressed.

With stores closed for months, customers couldn't try on items or return them in person. Returns are piling up, and shipping companies are struggling to keep up with demand, leading to long wait times for deliveries, some retailers said.

FedEx Corp. and the United Parcel Service Inc. are adding surcharges to some shipments in the U.S. to offset rising costs and manage a surge of packages.

For some companies, the situation is worse now than early in the pandemic, when online buying surged amid lockdowns, and businesses scrambled to get their customer-service teams working from home. That is because the volume of returns and overloaded shipping services led to delayed refunds, and shoppers who returned products weeks or months ago are now calling because they want their money back.

Wayfair had a large increase in sales volume in recent weeks that has added to the problems shipping companies face and led to record customer service demand, a spokeswoman said. The company is adding 500 additional customer-service workers to its U.S.-based team of 1,800.

Best Buy Co., its stores mostly closed since mid-March, said higher customer-call volumes from shoppers unable to get in-person help coupled with the disruption of sending customer-service employees to work from home, "meant we could not respond to customers as we would like and normally do."

"We're very sorry about that and are making it better every day," a Best Buy spokeswoman said.

Macy's, a frequent target of social-media outrage from disgruntled customers, apologized for long wait times on its customer-service website. A spokeswoman said service levels are improving and should be back to normal in the coming weeks.

Unable to keep up with online orders, IKEA apologized to shoppers and said it was recalling furloughed workers to help improve its customer support. Its website tells customers it is only accepting emails to cancel orders.

Salesforce.com Inc., a seller of customer-relationship management software, said weekly customer-service chatbot sessions for retailer and consumer-goods clients have increased nearly fourfold since the end of February. Chatbots use artificial-intelligence-based algorithms to understand and answer text or voice questions from customers.

Mr. Quinn, the Lululemon Athletica Inc. customer, received a confirmation from the retailer on April 18 that it received the returned yoga pants. Weeks later, when he hadn't gotten his money back, he said he contacted the company and reached a customer service agent who said the return should process in a few days, but it didn't. He said the customer-service wait time was 90 minutes, and emails went unanswered. He got his money back -- $180 for the two pants -- a few weeks ago.

"You'd think a company like them, as high end as they are, they would have that customer service piece figured out a little bit more," he said.

Lululemon's wait times are still longer than normal, and the company has extended its return-processing time to 15 to 20 days, up from 10 to 15 days, the company said.

The company, planning for increased call-center volume throughout the summer, said it would shift 500 store workers to its call center in addition to 300 workers already reassigned. Before the pandemic, Lululemon's had 300 call-center workers, said Celeste Burgoyne, Lululemon executive vice president of the Americas and customer relations.

The company's call-center volumes are two- to three-times the normal levels due to a spike in e-commerce sales and a high volume of mailed returns with fewer open stores, she said. April e-commerce revenue doubled compared with a year ago, the company said. Additionally, a distribution center in Washington state was shut down in March because of the pandemic.

Ms. Burgoyne said response times to the company's live chat and email are back to normal, while the call-center wait-times are minimal.

Matt Shapiro, a 39-year-old software developer from Seattle, is still waiting to hear from Best Buy about $400 of fraudulent purchases made on his credit-card account with the electronics retailer. The credit-card company refunded the money and issued a new card, but he wants to alert Best Buy to the problem and ensure that his current account is safe.

He said he has been unable to reach anyone on an online chat and when he last called, the recorded line said there was a 115-minute wait and offered that he leave a number to get a call back. That call never came, he said, and Best Buy hasn't answered his emails or tweets, which he said usually get a reaction from companies because he has some 40,000 Twitter followers.

"It's like the company doesn't exist," he said. "And they just have a website up and nothing underneath."

Write to Sharon Terlep at sharon.terlep@wsj.com