CHICAGO — Why do so many passengers get off the plane only to discover that their baggage did not make the trip with them?

American Airlines started asking that question with greater urgency a year ago, and its search for answers led to, among other problems, dirty printer heads.

Workers at American found that printers that produce adhesive tags for bags were often dirty. That made bar codes hard to read, leading to misdirected bags. Regular wiping of the printer heads helped, but even with a clean printer, the bar code readers are only about 90 to 92 percent accurate, said Denise P. Wilewski, manager of airport services for American here.

FWe never hit 100 percent — 90 percent is acceptable,” she said.

Airlines are fond of saying that they have a success rate of more than 99 percent in getting luggage to its destination along with its owner. And every big airline has stepped up efforts to improve its operations.