Please verify your request* Advertisement The Internal Revenue Service ended this year’s tax filing season with a backlog of more than 35 million unprocessed returns, a four-fold increase over 2019, National Taxpayer Advocate Erin Collins said in a report to Congress Wednesday. The 2021 tax season “was perhaps the most challenging filing season taxpayers, tax professionals, and the IRS have ever experienced,” Collins says in her report. The agency faced a “perfect storm” of challenges stemming from the Covid-19 pandemic and the relief efforts it was tasked with by Congress, including sending out three rounds of “stimulus checks,” or a total of about 475 million payments worth $807 billion. The agency also processed 136 million individual income tax returns and issued 96 million refunds totaling $270 billion.