WASHINGTON—U.S. services industry activity nudged up in September, but growth is being restrained by a persistent shortage of inputs and the resulting high prices as the pandemic drags on. The Institute for Supply Management said on Tuesday its non-manufacturing activity index edged up to a reading of 61.9 last month from 61.7 in August. A reading above 50 indicates growth in the services sector, which accounts for more than two-thirds of U.S. economic activity. Economists polled by Reuters had forecast the index falling to 60. A resurgence in COVID-19 infections, driven by the Delta variant, has delayed an anticipated upswing in demand for services like travel and other high-contact activities. Spending is steadily shifting from goods to services as the economy normalizes after being severely disrupted by the pandemic. The survey’s measure of new orders received by services businesses inched up to 63.5 last month from a reading of 63.2 in …