CHICAGO (CBS) — Southwest Airlines said it was expecting operations to be back to normal across the nation on Monday, a day after a computer glitch delayed hundreds of flights, but travelers were still being advised to print boarding passes at home or use a mobile boarding pass.

The Dallas-based airline said about 500 of its 3,600 flights on Sunday were delayed, after it was forced to use backup systems to check in travelers who did not have printed boarding passes or mobile boarding passes when they arrived.

Southwest has not said what caused the computer problems, but has said its system was not hacked.

Early Monday, Southwest said the problem had been resolved, and it expected its customer service systems to perform normally. Even so, it was advising passengers to check in online, and print boarding passes at home, or put their boarding passes on their mobile devices.

Sunday’s glitch forced Southwest agents to process tickets manually, or issue paper boarding passes, leading to crowded check-in lines.

“We have some additional work to do today to get bags delivered and some delayed or displaced Customers into open seats. We have teams working as quickly as possible to accomplish that,” the airline said Monday morning. “We continue to recommend that Customers check-in online or on our app before reaching the airport, and that they present a printed or mobile boarding pass to check luggage, proceed through security screening, and reach the boarding door of our gates.”

Chris Underwood said he found things running smoothly for Southwest on Monday morning at Midway International Airport, as he got ready to board a flight to Nashville.

“My boarding pass was too small, so I had to come back up and print another one, but other than that, it works out okay,” he said. “I printed one from home, but they said it was too small, so I had to go get another one, that’s all. No big deal.”

The airline said customers who need to change travel plans due to the computer problems on Sunday can do so at Southwest.com.

“It’s never too early to say thank you and to extend our apologies and we want to share those sentiments both with our hard-working Employees and our loyal and understanding Customers, whom we hope to welcome back for a better experience soon. We’ll continue to work individually with our affected Customers to make this right,” the airline said.

Monday morning, the vast majority of Southwest flights were listed as being on time at Midway International Airport.