iCal invites I send are not reaching recipient

Two out of three kids are receiving my shared calendar invitations or individual event invitations on their iPhones without any problems. The third one however, never receives the invite in his calendar app (or even an invitation via email but these are not "public" calendars either). I have noticed that when I add the first two, their names are highlighted in blue immediately. The other one stays gray and always shows "pending" on my iPhone or has a "?" on my Mac. Does not matter which device I send the invite from, he does not get it.

They all use their own active email addresses (same provider not that it matters) and passwords for their Apple accts and iCloud accts and I am using their Apple ID emails to send them the invites. The only way # 3 gets the invitations is if I send the invite to an iCloud email address that I just created for him. I'd prefer they only have one email address since they are under 18.

Why isn't # 3 getting the invites like the other two and anyone else I send to? Did I miss a setting somewhere in the initial setup of the iPhone? Maybe I should mention that # 3's phone is MY old iPhone 4 but it was wiped out and set up as a new phone with his Apple ID (Phone # 1 is a 5C and # 2 is a 4 also).