The issue is that the properties at fault are not coded as Hilton group hotels in whichever system the credit card companies use, resulting in no bonus points. Amex isn't using OCR to check for Hilton branding...
The four delinquent hotels I mentioned in my original reply were all 'genuine' Hiltons, described as Hiltons in my Amex statement (which at least made it easier for retroclaims), and at least 3 of them had always been Hiltons since their construction.
The retroclaim process was much easier before Amex abolished their secure messaging system, which ensured some accountability from customer service, unlike today's chat function.
FWIW all my retroclaims were processed by Amex, not HHonors.