Can’t AirDrop from iPhone to iPhone.

Thanks Lawerence. I was finally able to get this working via Apple's escalated high level phone support (solution below). Just to be clear, both phones were set to "everyone". Both phones can AirDrop to/from everyone else and other devices. Only one phone will refuse to see the other specific phone. It's a strange inconvenience we've just lived with. Usually just bouncing around by AirDropping between a third person or just using iMessage. But it's always perplexed us as to why both phones work perfectly with everyone else and each other EXCEPT for one direction (phone "b" to phone "a"). Phone "b" is just mysteriously invisible *just* to phone "a".

Solution: The problem appears to be a bug in the contact list. Deleting the contact of the "invisible" phone user and re-adding them did the trick. Even though both phones were set to "Everyone"; something in the contact list caused the user to be specifically "blocked". After over half a decade of this not working, it works! Who knew? Apple apparently!