The IRS failed to take all required steps for collecting unpaid taxes from people it can’t locate in over half the cases that investigators studied, potentially costing the government a pile of lost revenue, according to a federal report released Monday. The study does not estimate exactly how much money the IRS might collect if its workers were conducting all the research they are supposed to perform to find the taxpayers. The investigators wrote that in 2012, the IRS declared $6.7 billion in unpaid taxes to be uncollectable—involving nearly 483,000 tax returns—because it couldn’t find the taxpayers. Of that total,...