After two years of improved baggage handling, airlines slipped last year in getting passengers' checked luggage to arrive on time.
European airlines led the decline, while U.S. carriers actually saw a slight improvement.
Worldwide, 29.4 million bags last year didn't arrive on the same flight as their owners, according to SITA, an aviation communications and technology provider. That's 12.07 mishandled bags for every 1,000 passengers, a 6 percent increase over 2009.
The rise in baggage problems is attributed by SITA to more passengers flying, as well as major travel disruptions from severe winter weather and the ash cloud from an Icelandic volcanic eruption that grounded a large portion of European air traffic in April.
With an unprecedented 300,000 flight cancellations last year, there were bound to be some problems, said SITA spokesman Charlie Pryor.
"It puts huge pressures on the system that they simply weren't designed for," he said.
Difficulty with handling bags was greater in Europe. The mishandled rate there climbed to 12.6 bags per 1,000 passengers from 10.9 bags in 2009. In the United States, the rate fell to 3.57 bags per 1,000 passengers from 3.99 bags, according to the Department of Transportation.
Returning delayed bags to their owners cost airlines $2.95 billion last year, up from $2.5 billion in 2009.
Despite the slip, the airlines are doing substantially better than 2007. Back then, 42.4 million bags were delayed at the rate of 18.86 mishandled bags per 1,000 passengers.
SITA credits the overall improvement to a combination of better tracking and fewer bags being checked because of relatively new fees. U.S. airlines alone are collecting more than $3.3 billion a year in such fees.