...to file a “Missing Mail” claim with the USPS.

Doesn’t matter if it was Priority Mail or insured, as long as you have a 1st Class tracking number, it makes people look at the problem.

https://www.usps.com/help/missing-mail.htm

By filing a claim on-line, it makes the involved Post Offices look at the issue and respond to you - either good or bad news.

Every time I have started one of these inquiries, the package begins to move - even when the Post Office apologized for losing the package!

Worth a shot.