The USCIS Ombudsman, an independent DHS office that provides recommendations for resolving individual and employer problems with USCIS, recommends contacting its office when an I-797 receipt is not issued within the time frames on the USCIS website. See the following response below on this issue from the CIS Ombudsman webpage:
Posted Receipting Times – USCIS posts processing dates for which applications are now being receipted. However, these seem to be incorrect because customers who filed well before these dates still do not have receipts. Moreover, applications received after the posted dates have been sent receipt notices. Can you explain why?
Response: We understand USCIS is receipting the majority of cases according to the weekly “Receipting Update,” found at www.uscis.gov. If you have not received your receipt by the date indicated on the “Receipting Update,” please send an email to the Ombudsman at firstname.lastname@example.org. Note that if you send an email on this or any issue to the Ombudsman, please be aware that we do not adjudicate cases. You will still need to follow USCIS instructions to address your case. In addition, if you want the Ombudsman to research your particular situation, please submit a case problem by using Form 7001, available at www.dhs.gov/cisombudsman.