So you’ve racked up a bunch of frequent flier miles or loyalty points or whatever your preferred airline calls them, but can you actually use those rewards to book free travel when you want? For some U.S.-based carriers, the answer ranges from “almost definitely” to “good luck.”

Airline research firm IdeaWorks Company has released the results of its annual Switchfly Reward Seat Availability Survey which looks at a wide variety of carriers to see how much flexibility you have in using your rewards miles.

Southwest‘s Rapid Rewards once again led all U.S. carriers (and tied with airberlin for the top spot overall) with 100% of the tester’s attempted travel dates available for booking with rewards points.

JetBlue was the #2 U.S. airline (tied for #5 overall), with an availability rate of 87.1%, down nearly 6 points from the previous year.

Alaska Air Group, which runs Alaska Airlines and Horizon, showed a huge 20 percentage point year-over-year improvement in rewards availability, leaping up to 80%. Alaska was followed by United with 75% availability.

American Airlines‘ 67% availability rate isn’t impressive, but it is 12 percentage points better than last year. Meanwhile, Delta had the lowest availability rate of American carriers, with only 57.9% of rewards booking queries available.

“Consumers expect something of value from brands in return for their business and for their loyalty, and they want and expect offers that are relevant reflections of themselves,” said Switchfly CEO Daniel Farrar. “The key takeaway is that airlines can’t afford to allow a single customer to have a bad experience anywhere in the booking or redemption path.”

To test for availability, IdeaWorks attempted to book travel on 280 specific dates for each airline, using only rewards travel. Each itinerary in the test included a Saturday night stay. The availability rate reflects the percentage of selected travel dates for which the testers were able to use their rewards miles.

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