Adidas AG notched a big win when it signed a deal in January to sponsor University of Miami sports. The school had been rival Nike Inc.’s turf for decades, a perfect place to show that Adidas’s new U.S. chief was breaking its long slump in America.

But many fans of “The U” balked, starting anti-Adidas chatter on social media. Miami’s coaches seemed excited, says Mark King, Adidas’s new North America president. “But at the kid level,” he says of students, “they’re like, ‘Why would you sign with Adidas?’ ”

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