The Department of Transportation is looking into Southwest Airlines’ massive number of cancellations. The airline has been plagued with bad luck, as a winter storm hit its biggest hubs in Chicago and Denver.

Senators are calling on Southwest to compensate passengers for ruined travel plans. They want the airline to offer rebooking, ticket refunds and hotel, food and transportation reimbursements.

Weather

A punishing winter storm caused widespread flight cancellations this week, and Southwest has been hardest hit. On Wednesday, it canceled about 2,500 flights, or more than half of its planned flights, according to flight-tracking service FlightAware.

Airline scheduling is a complicated process, taking into account union rules and airline policies, says Kathleen Bangs, a former commercial pilot and spokeswoman for FlightAware. And although airlines say they are restoring normal operations, the cancellations are likely to continue for days to come.

For travelers like Katie McNamara, a Brooklyn art director visiting family in Mississippi for the holidays, finding another way home isn’t easy. She was forced to stay an extra day because of her canceled flight and now has no other options until Jan. 31. The airline has set up a self-service portal to help passengers get refunds for hotel rooms and ground transportation. But requests are being reviewed on a case-by-case basis. The DOT has criticized the company for “acceptable rates of flight delays and cancellations and inadequate customer service.” It will examine whether Southwest has fulfilled its customer service plan and other requirements.

Staffing

As Winter Storm Elliott wreaked havoc on the nation’s airports, Southwest stood out from the rest of the airline industry, drawing unwanted attention from Washington. The Dallas-based carrier’s systemic disruptions, caused by a combination of bad luck and poor planning, frustrated travelers who spent their holiday stuck in Austin-Bergstrom International Airport or separated from stacks of unclaimed luggage at Denver International Airport.

The company attributed the problems to a combination of severe weather and staffing issues. The airline’s vice president of ground operations circulated a memo on Dec. 21 declaring a “state of operational emergency” at the airline’s hub in Denver after an unusually high number of ramp workers called in sick for afternoon and evening shifts, reports say.

Airline scheduling is a complicated system that takes into account union rules, government regulations and airline policies, experts say. And the way it works, a flight’s status can change in minutes from safe to canceled, which leads to planes and crews being pushed around the country in a frantic attempt to keep them on schedule.

Electronics

While Southwest Airlines has been able to get flights going again, there are still plenty of people who have not had any luck getting home for the holidays. If your travel is affected, check with your travel insurance to see if you can have hotel and food expenses reimbursed.

Southwest is making some improvements to its fleet, such as introducing power ports at every seat and upgrading in-flight Wi-Fi on existing planes to a newer generation with faster bandwidth. That should be available to most passengers by the fall.

Southwest is unique among major airlines, following a point-to-point model that allows its planes to fly directly from smaller cities and regions to larger destinations without having to stop at hub airports like Denver or New York City. This allows it to cut travel times by a significant amount compared to other carriers. That model also requires fewer employees. However, the airline has seen significant staffing problems this month during the winter weather disruptions.

Computers

Last year, Southwest had to cancel nearly two-thirds of its flights after a massive winter storm caused the airline to buckle under the weight of an antiquated computer system. The airline was forced to offer refunds and food vouchers for irate customers after the chaos and lost $75 million in revenue.

Its service meltdown occurred after a winter storm and before the start of the summer travel season, when passengers can expect to see more flight delays due to weather and illness. While other airlines experienced some service disruptions during the storm, Southwest’s problems were much more serious because of an old internal system that handles crew scheduling.

That system is called Skysolver and it can only handle so many flight cancellations before it grinds to a halt. The software has been around for decades, but the company hasn’t spent money to keep it up to date or replace it with something more modern. As a result, the system is often overwhelmed during times of trouble and staffing is disrupted.