Customers of other airlines including United have also encountered frustrations with getting refunds.
Delta issued a statement saying if it cancels a flight, customers will first be invited to rebook. If they don’t, “the value of the ticket becomes an eCredit…. Alternatively, eligible customers are able to request a refund.”
Those who search Delta's website can find an online form to apply for a refund.
While Delta normally gives refunds within seven days for credit card purchases, it says it now could take up to 21 business days.
Delta has moved to make flight credits more appealing by extending some credits through May 31, 2022. Such credits are available to travelers who canceled trips booked for March, April or May 2020.
The DOT said it has received an increasing number of complaints from travelers "who describe having been denied refunds for flights that were cancelled or significantly delayed.
"In many of these cases, the passengers stated that the carrier informed them that they would receive vouchers or credits for future travel," which are not readily usable because of flight cuts, according to the DOT.
“The longstanding obligation of carriers to provide refunds for flights that carriers cancel or significantly delay does not cease when the flight disruptions are outside of the carrier’s control,” said the DOT, emphasizing “the fact that the cancellation is through no fault of the passenger.”
The DOT said it would allow airlines to comply with refund requirements before taking further action.
In order to comply, airlines must contact passengers provided vouchers for flights the carrier canceled or significantly delayed, and to notify those passengers they have the option of a refund.
Airlines must also make clear in their policies that they provide refunds if the carrier cancels a flight or makes a significant schedule change.
And they must review the circumstances when refunds should be made with reservation agents, ticket counter agents and others in customer service.
The DOT said its aviation enforcement office “will monitor airline policies and practices and take enforcement action as necessary.”