A new survey has some good news for Atlanta frequent fliers who have struggled to redeem their miles for flights in the past: Delta Air Lines has actually increased its availability of reward flights, and Southwest Airlines has the best availability of U.S. airlines, the survey found.

Atlanta-based Delta ranked 16th out of 25 carriers around the world in a Switchfly reward seat availability survey by consulting firm IdeaWorksCompany. Delta improved its reward seat availability by 10.7 points year-over-year, with 68.6 percent of queries resulting in reward seats, according to the annual survey.

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Credit: Kelly Yamanouchi

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Credit: Kelly Yamanouchi

The survey results show improvement, but still mean that some searches for reward seats come up short. Delta SkyMiles members have voiced frustration with some of the airline's changes to its SkyMiles program that make the cost for a flight in miles more variable.

Delta's incoming president Glen Hauenstein said earlier this year that the company wants to encourage its customers to spend miles instead of saving them for free flights, by finding "new and creative ways for our customers to burn those miles" such as using miles to buy a bottle of champagne in a Sky Club.

Southwest, the second-largest carrier in Atlanta, had a 100 percent score in the survey, putting it in the No. 1 spot.

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Credit: Kelly Yamanouchi

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Credit: Kelly Yamanouchi

In the airline industry overall, the survey showed improvement in reward availability.

Customers "know when their loyalty programs are offering them a real value and when they are not delivering," said Daniel Farrar, CEO of travel services firm Switchfly, in a written statement.

Availability of reward seats for long-haul flights is also improving, the survey found. Delta's long-haul reward availability improved 24.3 points, with 55.7 percent of queries yielding reward seats.