UPDATED | October 11, 2015 7:18 PM

Follow CBSDFW.COM: Facebook | Twitter

DALLAS (CBSDFW.COM) — Technical issues that were causing delays for some Southwest Airlines flights Sunday morning continued into the afternoon.

READ MORE: 'My Nerves Are Still Rattled': Passenger Aboard Amtrak Train Talks Crash

Southwest personnel had to manually process customers arriving at airports, according to a statement from Southwest management. As of 10 a.m., less than 100 of the 3,600 flights scheduled for Sunday had been delayed, but that number increased to about 450 by late afternoon.

The Dallas-based company asked travelers to arrive at least two hours before their scheduled departures. It is also asking customers to use airport kiosks to print boarding passes and tags for luggage.

Representatives for Southwest did not say what caused the problem or how long it would take to resolve. Spokesman Brad Hawkins said there was “absolutely no indication now” that the problems were the result of hack.

E.J. Schultz, a reporter for Ad Age who was taking a Southwest flight from Chicago’s Midway International Airport, said the airline was telling people at the gate that travelers with paper boarding passes were fine. But those who had downloaded their tickets onto their mobile phones were told they had to stand in line, he said.

Schultz said he didn’t understand why Southwest didn’t announce that people should print out their boarding passes at home before getting to the airport.

“If everyone had done that, it would’ve saved so much time,” he said.

Schultz said there was a line of about 50 people at the Southwest gate. His flight took off roughly 15 minutes after its scheduled departure time of 4:30.

READ MORE: Man, Pregnant Woman & Baby Killed In Crash Along Highway 360; Police Investigating

The long lines at check-in may mean some passengers didn’t make their flights.

Emily Mitnick, who was flying to Detroit from Denver International Airport, said she missed her 10 a.m. flight, even though she parked her car around 8 a.m. She estimated that about 1,000 people were on line at the check-in for a boarding pass. When she went downstairs to the curb-side check-in, she said there were about a couple hundred people in line there as well.

By the time she got in line to go through security, it was around 10:15 a.m.

“The clock was ticking and the flight took off,” said Mitnick, who was trying to get to Detroit through a different flight to Chicago.

In a statement, Southwest said it was still having “intermittent” technical issues on its website, mobile app and in its phone centers and airports check-in systems. It said that while it is working on the issues, workers at airports are helping customers with their itineraries.

Last month, American Airlines experienced computer problems that prevented passengers from checking in and briefly halted flights on select routes. Airline officials said at the time that they fixed the problem after less than two hours, and that there was no indication that its system had been hacked.

In July, hundreds of United Airlines flights were delayed after the airline experienced computer problems for the second time in just over a month. A United representative said at the time that the glitch was caused by an internal technology issue, and not an outside threat or hacker.

MORE NEWS: Flash Flooding: Second Body Recovered After Vehicle Swept From Texas Bridge

(©2015 CBS Local Media, a division of CBS Radio Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)