Once again, a technical glitch at a major airline has led to delayed flights -- and frustration -- for passengers traveling around the country. On Sunday morning, Southwest Airlines reported “a system issue” that was affecting the company’s digital check-in systems.

Many passengers reported on social media that airline employees were manually processing each customer, causing long lines and missed flights. Customers were being advised to arrive at the airport two hours early. In a statement, the company confirmed it was experiencing a systemwide technical glitch, but it has not yet revealed the cause.

“We apologize for this morning's technical issues,” the Dallas-based airline tweeted early today. “We are working to restore service to our customers, and we appreciate your patience.”



A company spokesman, Michelle Agnew, told NBC News that about 100 flights at various airports around the country had been delayed because of the glitch. “While teams work diligently to enhance the performance of our technology, our airport-based employees are working with customers on their individual itineraries,” Agnew said.

This isn’t the first time in recent months that a software glitch has thrown a wrench into travelers’ plans. In September, a glitch at American Airlines grounded flights at three of its busiest airports. And in July, a system error at an air traffic control center in northern Virginia disrupted flights along the East Coast.

Not surprisingly, many passengers shared their aggravation online. “This little glitch is causing major problems,” wrote Susan Watson on Twitter. 

"We share your frustration and are doing our best to resolve these issues quickly," the company responded.