Federal and state returns e-filed through Pro-series and accepted 07/02/2020 for year 2019.  Document stamp of acceptance received for federal and two state returns.  State refunds received.  Federal refund not received.  IRS website continued to show "processing".  

IRS issues letter dated 02/15/2021 indicating that return was not received and is delinquent.  Sending in paper return along with document stamp supporting electronic filing.

Is there any way to understand how IRS systems failed to process an e-filed return?