Irs claims they never received extension request I sent in April 9th. I was stupid and misplaced the receipt from USPS so I have no proof either. Can I do anything?

My wife and daughter didn't have ss at the time of filing so had to file for an extension. They adjusted my return by a lot because they claim they never received the extension request. I remember going to the USPS post office and making sure I did priority delivery too.