I suspect the two people I want to invite have old Macs and iPads stashed in a closet somewhere that are preventing the invitations I send from my home from going through. anyone have a fool-proof system to try to get them access?

I think I've tried almost everything, including resetting both my home and their home. I was able to invite one family member to my home who lives with me, but the other two who live in a different house with another apple home setup never get the invites.