Could this have something to do with their message settings?
On PS4, you can set it up so you don't receive any incoming messages from people. I have noticed something similar when trying to send a clan invite to someone with this option turned on.
I was also wondering if parental controls would block an incoming clan invitation the same as it would an incoming message.
I don't think it's related to that, as my attempts to send, cancel and re-send invites (after I discovered how to confirm when one was sent or not by looking for the message to appear in Clan Chat) were all done within seconds of each other (i.e. failed invite, resend, failed invite, resend, success). For what you're suggesting to be the issue, the people not getting invites would have had to modify those message settings at some point between the first failed invite and the future successful one seconds later.