Hundreds of Southwest Airlines flights were delayed or canceled again on Wednesday as the company sought to resolve disruptions from earlier in the week amid a pickup in summer travel.

The headaches for Southwest, which is widely credited for pioneering the low-fare airline business model, began on Monday night, when a problem with a weather data supplier prevented the airline from safely flying planes. The issue was resolved within hours, but on Tuesday the airline suffered its own technological problems, resulting in half of its flights that day being delayed and many being canceled, according to FlightAware, a flight tracking service.

Spillover from that episode caused Wednesday’s problems, the airline said. About 10% of Southwest’s flights were canceled and an additional 19% were delayed by midafternoon, according to FlightAware.

“While our technology issues from Tuesday have been resolved, we are still experiencing a small number of cancellations and delays across our network as we continue working to resume normal operations,” Dan Landson, a Southwest spokesman, said in a statement.

Southwest said on Tuesday that it was having problems with “network connectivity.” Landson said that those troubles were unrelated to the weather data problems from Monday and that there was no indication the airline’s computer systems had been breached or hacked.

The flight disruptions came at a critical time for a company celebrating its 50th year.

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After seeing demand for travel plummet during the pandemic, airlines are experiencing a rebound as coronavirus cases in the United States fall and vaccinations increase. More people are flying for summer vacations, and businesses and states and cities are opening up. The governors of California and New York on Tuesday lifted most pandemic restrictions. The Transportation Security Administration screened more than 2 million people at airports on Sunday, more than at any point since March 2020.

Southwest was fortunate in that the delays and cancellations occurred during a part of the week that is typically less busy, but thousands of customers were still affected.

One passenger, Efia Brown, and her daughter were trying to return to their home in St. Louis from Los Angeles on Monday when Southwest Airlines canceled a connecting flight while they were waiting at the airport in Denver. Brown’s daughter became sick on their earlier flight, and after the airline notified them of the cancellation, Brown booked another for 9:30 p.m. on Tuesday. But her daughter was admitted to a hospital on Tuesday after vomiting several times at the airport gate. The family is still in Denver.

“It’s very disappointing because this is the only airline I fly with,” Brown said. “When this happened, it was so frustrating, I had to hold back my tears.” Another traveler, Azavier Jackson, 29, said she was disappointed with Southwest, which she was flying for the first time. Her flight from Los Angeles to Charlotte, North Carolina, was delayed, as was her return trip home on Monday. Jackson’s connecting flight from Dallas to Los Angeles kept getting pushed back on Monday night, and she left the airport at 6 a.m. on Tuesday. Her biggest frustration was that the airline didn’t inform her of an earlier flight that she could have taken.

“It’s not humane to make us wait in the airport that long,” Jackson said. “There was terrible communication. It felt like they were saying, ‘Just deal with it.’”

The disruptions have been difficult for Southwest’s staff, too, said Lyn Montgomery, president of Transport Workers Union Local 556, which represents Southwest’s flight attendants. The airline’s management has been providing regular updates to flight attendants, and Southwest worked to provide accommodations for those who were stranded, she said, adding that such outages happen.

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“We are all victims to computers and computer issues,” she said, “and when it happens at an airline, it tends to be massive.”

Like other airlines, Southwest suffered severe losses as the pandemic forced many people to cancel travel plans and stay home for months on end. But the airline, which carried more passengers than any other in the United States in 2019, fared better than most.

Southwest, which reported an annual loss in 2020, its first in nearly 50 years, entered the pandemic with less debt than any other major U.S. airline. And because it flies largely within the United States, it has been relatively unaffected by the sharp drop in international travel. As a result, Southwest reported a modest profit in the first three months of the year, the first major U.S. airline to make money since the pandemic began.

So far this year, the airline has announced it was expanding an order for Boeing’s 737 MAX by 100 planes and recalling all flight attendants who had taken extended leaves of absence in time for the seasonal increase in travel during the summer. In May, Southwest said it was seeing improvements in demand, and this month it said it would buy even more MAX jets.

But the rebound hasn’t been without its difficulties. Business travel has yet to recover meaningfully, and the number of people traveling is still about 20% below 2019 levels, according to TSA data. Like other airlines, Southwest has also had to contend with a surge in unruly and sometimes violent behavior from passengers, leading it to pause plans to restart selling alcohol on its flights.