In some airline loyalty programs, only 8% of frequent flyer miles are redeemed. Here's why it might be getting harder to score a free ticket.

Many travelers are a member of at least one airline frequent flyer program — and lots are members of multiple airline loyalty programs. And the membership numbers keep growing, as well as the accumulated earned mileage waiting to be redeemed. And therein lies the promise — and the problem.

The modern frequent flyer program began more than 40 years ago, a little more than two years after airline deregulation unleashed greater competition for passengers. 

American Airlines launched first among the big carriers in 1981, quickly followed by Delta and United. Initially, American's membership was by invitation only. Today it has 115 million members, closely followed by United  — 100 million-plus — and Delta, 92 million.