Refund delay why?

According to this IRS update as of April 30th:    

The IRS is now opening mail within normal timeframes. The IRS has also made significant progress in processing prior year returns. As of April 23, 2021, we had 1.3 million individual tax returns received prior to 2021 in the processing pipeline. Including current year returns, as of April 23, 2021, we had 16.9 million unprocessed individual returns in the pipeline. Unprocessed returns include those requiring correction to the Recovery Rebate Credit amount or validation of 2019 income used to figure the Earned Income Tax Credit (EITC) and Additional Child Tax Credit (ACTC). This work does not require us to correspond with taxpayers but does require special handling by an IRS employee so, in these instances, it is taking the IRS more than 21 days to issue any related refund. If, as a result, a correction is made to any RRC, EITC or ACTC claimed on the return, the IRS will send taxpayers an explanation. Taxpayers are encouraged to continue to check Where’s My Refund? for their personalized refund status and can review Tax Season Refund Frequently Asked Questions.  

How long you may have to wait: The IRS understands the importance of timely processing of tax returns and refund issuance. We are processing returns received over the summer and fall in 2020 due to the extended July 15, 2020 tax filing due date. While the majority of 2019 refund returns have been processed, in some cases, we are processing tax returns that were mailed with a payment even though payment associated with these returns have been processed by the IRS. However, we are rerouting tax returns and taxpayer correspondence from locations that are behind to locations where more staff is available, and we are taking other actions to minimize any delays. Tax returns are opened in the order received. As the return is processed, it may be delayed because it has a mistake including errors concerning the Recovery Rebate Credit, is missing information, or there is suspected identity theft or fraud. If we can fix it without contacting you, we will. If we need more information or need you to verify that it was you who sent the tax return, we will write you a letter. The resolution of these issues depends on how quickly and accurately you respond, and the ability of IRS staff trained and working under social distancing requirements to complete the processing of your return.  

What you should do: If you filed electronically and received an acknowledgement, you do not need to take any further action other than promptly responding to any requests for information. If you filed on paper, check Where’s my refund? If it tells you we have received your return or are processing or reviewing it, we are processing your return, but it may be under review. We’re working hard to get through the backlog. Please don’t file a second tax return or contact the IRS about the status of your return.