"It took USA TODAY 13 attempts recently before it could book a free seat at the minimum mileage level on a Delta flight.
Despite changing arrival and departure dates and the arrival city, USA TODAY repeatedly struck out trying to book a free flight at the lowest mileage threshold from New York to six European cities in August, a peak travel month.
USA TODAY also was unable to get a free seat for the minimum number of miles for flights during two off-peak months: New York to Los Angeles in September and New York to Omaha in October.
Delta's website sometimes showed dates when free seats at the lowest mileage levels were supposedly available, but only free flights requiring higher mileage appeared when clicking on the dates.
A telephone agent for Delta's SkyMiles program said the website wasn't as up to date as the changing seat inventory in the airline's reservations system.
Delta spokeswoman Chris Singley says the airline on March 13 upgraded the award calendar on its website "to improve its accuracy." USA TODAY, though, tried to book its free flights four days later and found them inaccurate.
Singley says that in late March more than 20% of seats remaining for April and May flights were available at the lowest mileage level.
Demand is "very high" for free seats on summer flights, particularly with average airfares continuing to rise, she says. "Just as the lowest fares will sell out quickly, so do award seats at the lowest redemption levels."