Despite the low numbers for some airlines, the overall availability of seats increased slightly over last year, to 68.6 percent from just over 66 percent.

FORBES: Delta, U.S. Airways Rank Worst for Frequent Flier Options

Mr Chivers said he was encouraged by reported improvements in the survey in areas such as availability and comfort of seats on trains.

BBC: Rail fare 'dissatisfaction' grows, survey suggests

Delta and US Airways have scored at the bottom for award seats all four years of the survey, though both have upped their availability.

WSJ: Frequent-Flier Miles: Best Airlines for Cashing In

Consumers can sometimes find better award availability by calling airlines and getting an experienced agent to hunt for seats and check the inventory of airline partners.

WSJ: Frequent-Flier Miles: Best Airlines for Cashing In