Switchfly says Southwest and the German discount carrier Air Berlin had reward seats available on every flight checked. They were followed byVirgin Australia, Air Canada and Singapore.
The three largest airlines in the U.S., American, Delta and United all increased their reward seat availability.
Meanwhile, airline showing the biggest improvement in open reward seats was Alaska. Switchfly found 80 percent of Alaska Airline flights had seats open for bookings under the company's Mileage Plan program.
Read MoreAirline fees not going away: Delta CEO
Caroline Boren with Alaska Airlines says the carrier has been more aggressive freeing up seats for frequent fliers, even if that means taking away seats that might be sold.
"There's a short term financial benefit to selling a seat, but there's a long-term benefit to having loyal customers if you open seats for frequent flier rewards," she said.
Alaska's improved results in the latest Switchfly survey come one day after the airline's Mileage Plan scored highest in J.D. Power's latest report on airline reward program customer satisfaction.
Even with this study showing more seats are open for reward redemption, many travelers question if frequent flier programs are rewarding enough members. Both Delta and United have changed their programs in the last year so miles are accrued based on how much money people spend on tickets, not on how far they fly.