The Switchfly survey found some carriers, namely Southwest and the German discount airline Air Berlin, had reward seats available on every flight.
"I think they [Southwest] have made a conscious effort to manage their program better in terms of being concerned about reward seat availability and really increasing reward seat availability at the expense of selling that seat to a fare-paying customer," Sorensen said.
Others, including American Airlines and Chile-based Lan, offered open seats on closer to 50 percent of surveyed flights, putting them at or toward the bottom of the list. The survey found open reward seats on just 56.4 percent of the American Airlines flights it inquired about, down 10.7 points compared with last year.
Sorensen said American's AAdvantage frequent flier program, which has 100 million members, may not have received as much attention from the company as it should have, as American and US Airways were preoccupied with their merger over the last few years.
"American achieved a herculean effort last year by merging two massive airlines with really no major problems," he said. "I think they devoted all their energy into that and I think that projects like the AAdvantage frequent flier program somewhat took a backseat in terms of innovation."
American Airlines spokesman Josh Freed says the number of AAdvantage redemptions climbed 5 percent last year.
"The percentage of our customers traveling on award tickets rose as well," he said. "Also, the number of MileSAAver redemptions rose last year, showing that people are finding availability at our lowest-mile award levels."