With one of its biggest corporate sponsors pulling back from a multiyear underwriting commitment, NPR has an uphill climb to rebuild its sponsorship revenues from 2011, when the network’s sales reps reeled in enough deals to set a new earnings record. High turnover — or “churn” — among its corporate clients, and the pullback of companies that spend the most money on advertising, cut into NPR’s bottom line in the fiscal year that just closed. After netting $2.4 million in profits in 2011, largely on robust sponsorship sales, the network aimed high for 2012. But sales fell far short of projections. For fiscal year 2012, which ended on Sept.