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McDonald's Corp., battling back from recent earnings disappointments, is putting unusual emphasis on a longtime challenge: getting its far-flung workforce to provide service with a smile.

The fast-food giant, whose restaurant sales in the U.S. began to slip last year, is pushing franchisees to improve staffing and service amid mounting complaints about rude employees.

In a webcast McDonald's executives held with franchise owners last month, the company said 1 in 5 customer complaints are related to friendliness issues "and it's increasing," according to a slide from the presentation reviewed by The Wall Street Journal.

The webcast identified the top complaint as "rude or unprofessional employees."

One slide said that complaints about speed of service "have increased significantly over the past six months." Another mentioned that customers find service "chaotic."

"Service is broken," said a slide from part of the webcast delivered by Steve Levigne, vice president of business research for McDonald's USA.

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