Referenews

Citing in part early-morning fog in Chicago and heavy snowfall in Denver, Southwest canceled roughly 130 flights Monday, or about 3% of what it had planned, according to FlightAware. The airline had scrubbed many of those flights proactively before Monday in anticipation of the weather. 

“We’re operating a normal schedule today and are pleased with the operational performance over the past few days,” a spokeswoman said. 

Flight disruptions across the industry mounted Monday afternoon.

The Federal Aviation Administration slowed the volume of flights into Florida’s airspace because of an air traffic control computer issue at a Miami facility that oversees flights across a swath of the state. Flights faced an average delay of more than two hours, according to an FAA notice.

The agency said the issue, which impacted a system at a facility that handles en route traffic, has been resolved. 

“The FAA is working toward safely returning to a normal traffic rate in the Florida airspace,” it said.

In total, more than 600 flights to, from or within the U.S. were canceled as of Monday afternoon, according to FlightAware.

Southwest said it was monitoring the impact of the slowdown on its operations, and said it would work to minimize customer disruptions.

Other issues affected air travel over the weekend.

San Diego International Airport has run short on fuel, after a segment of a pipeline that serves the area was shut down in late December because of a leak. Some flights from that airport have had to make pit stops to fill up, airlines said.

Kinder Morgan Inc.,

the pipeline operator, said it expects the pipeline to return to service Tuesday.

Airports in Houston and Dallas faced delays Monday because of storms, according to FAA advisories.

The flare-ups around the country are a test for Southwest, which is under particularly intense pressure to deliver a smooth travel experience after it struggled with the rolling fallout of a severe storm and then scrapped a swath of its schedule for three days in an effort to reboot itself.

While that storm created problems for all airlines, Southwest canceled far more flights and was much slower than others to recover. Executives of the airline have said the scheduling system used to revise crew schedules after disruptions was overwhelmed by the volume of changes required. The airline canceled nearly 16,000 flights from Dec. 22 through Dec. 29.

The airline, the largest carrier of U.S. domestic passengers, faces the task of reassuring its customers amid intensifying scrutiny by lawmakers and regulators. Transportation Secretary

Pete Buttigieg

has said the agency will be paying close attention to Southwest’s efforts to compensate affected customers, and Democratic lawmakers have suggested that the airline’s troubles are a sign the government needs to pursue more aggressive measures aimed at protecting passengers.

Southwest executives have said work to upgrade some of its balky systems was already under way, but the meltdown will prompt a deeper review. Chief Operating Officer

Andrew Watterson

told employees in an update Saturday that he and Chief Executive

Bob Jordan

plan to launch a full postmortem analysis of the problems, starting Tuesday. 

“We plan to have a clear-eyed, transparent view of what went wrong while still fresh in everyone’s memories,” Mr. Watterson said.

Southwest has apologized for the difficulties and pledged to refund flights and reimburse customers’ expenses during the disruptions. 

“There’s no way to tell everybody, or to ask you, to turn the page on this event, because it’s so significant, and we’re going to be working on healing with our customers and each other for a long time,” Mr. Jordan told employees in a message Sunday.

The airline recruited volunteers over the weekend to help process customer refunds and sort through the deluge of missing bags in an effort to return them to customers. 

Some customers who were separated from bags last week said they are still waiting for them. 

James Nolan said he has used AirTags to watch as his family’s three bags have made their way around Denver International Airport since their Dec. 26 flight to Sacramento, Calif., was canceled. One day they were sitting outside the terminal C gates, then moved to a cargo yard, then to a maintenance hangar. He said he returned to the airport twice in search of the bags and made several calls, without success. When he and his family took a rebooked flight to Sacramento on Dec. 30, the bags weren’t on the flight. 

“Unfortunately, we do not have a resolution,” Mr. Nolan said Monday.

Southwest has said it is working to get bags back to customers, including using

United Parcel Service Inc.

and

FedEx Corp.

to transport lost luggage. 

Dan Adams, who has been trying to get his luggage back since his family opted not to take their flight from Atlanta to Dallas on Dec. 24 because of an hourslong delay, said Monday that he had gotten a notification from

FedEx

that two of his three bags should arrive this week. On Monday, they retrieved the third bag—his daughter’s—after driving to the airport in Atlanta, where employees were able to find it after looking for about an hour, Mr. Adams said.

—James R. Hagerty contributed to this article.

Write to Alison Sider at alison.sider@wsj.com